[plugins] Add init hook to plugins. Add ruby gem bin to path DEV-1271 (#389)

## Summary

* Adds init hooks to plugins using same structure as devbox.json 
* Ensures ruby gem bin path is in PATH. Adding in an init hook ensures
we respect user provided `$GEM_HOME` (currently it's not possible to
override plugin env vars, but once we allow env vars in `devbox.json`
they will take precedence.

cc: @mohsenari @Lagoja 

## How was it tested?

```
devbox add ruby nginx
devbox shell
gem install rails
which rails
```

Signed-off-by: Mike Landau <mikeland86@gmail.com>
Co-authored-by: Greg Curtis <greg.curtis@jetpack.io>

Author: mikeland73

internal/impl/config.go

@@ -4,17 +4,15 @@
 package impl
 
 import (
-	"encoding/json"
-	"fmt"
 	"os"
 	"path/filepath"
 	"strings"
-	"unicode"
 
 	"github.com/pkg/errors"
 	"go.jetpack.io/devbox/internal/boxcli/usererr"
 	"go.jetpack.io/devbox/internal/cuecfg"
 	"go.jetpack.io/devbox/internal/debug"
+	"go.jetpack.io/devbox/internal/impl/shellcmd"
 	"go.jetpack.io/devbox/internal/planner/plansdk"
 )
 
@@ -36,8 +34,8 @@ type Config struct {
 	// Shell configures the devbox shell environment.
 	Shell struct {
 		// InitHook contains commands that will run at shell startup.
-		InitHook ConfigShellCmds             `json:"init_hook,omitempty"`
-		Scripts  map[string]*ConfigShellCmds `json:"scripts,omitempty"`
+		InitHook shellcmd.Commands             `json:"init_hook,omitempty"`
+		Scripts  map[string]*shellcmd.Commands `json:"scripts,omitempty"`
 	} `json:"shell,omitempty"`
 
 	// Nixpkgs specifies the repository to pull packages from
@@ -94,114 +92,6 @@ func WriteConfig(path string, cfg *Config) error {
 	return cuecfg.WriteFile(path, cfg)
 }
 
-// Formats for marshalling and unmarshalling a series of shell commands in a
-// devbox config.
-const (
-	// CmdArray formats shell commands as an array of of strings.
-	CmdArray CmdFormat = iota
-
-	// CmdString formats shell commands as a single string.
-	CmdString
-)
-
-// CmdFormat defines a way of formatting shell commands in a devbox config.
-type CmdFormat int
-
-func (c CmdFormat) String() string {
-	switch c {
-	case CmdArray:
-		return "array"
-	case CmdString:
-		return "string"
-	default:
-		return fmt.Sprintf("invalid (%d)", c)
-	}
-}
-
-// ConfigShellCmds marshals and unmarshals shell commands from a devbox config
-// as either a single string or an array of strings. It preserves the original
-// value such that:
-//
-//	data == marshal(unmarshal(data)))
-type ConfigShellCmds struct {
-	// MarshalAs determines how MarshalJSON encodes the shell commands.
-	// UnmarshalJSON will set MarshalAs automatically so that commands
-	// marshal back to their original format. The default zero-value
-	// formats them as an array.
-	//
-	MarshalAs CmdFormat
-	Cmds      []string
-}
-
-// AppendScript appends each line of a script to s.Cmds. It also applies the
-// following formatting rules:
-//
-//   - Trim leading newlines from the script.
-//   - Trim trailing whitespace from the script.
-//   - If the first line of the script begins with one or more tabs ('\t'), then
-//     unindent each line by that same number of tabs.
-//   - Remove trailing whitespace from each line.
-//
-// Note that unindenting only happens when a line starts with at least as many
-// tabs as the first line. If it starts with fewer tabs, then it is not
-// unindented at all.
-func (s *ConfigShellCmds) AppendScript(script string) {
-	script = strings.TrimLeft(script, "\r\n ")
-	script = strings.TrimRightFunc(script, unicode.IsSpace)
-	if len(script) == 0 {
-		return
-	}
-	prefixLen := strings.IndexFunc(script, func(r rune) bool { return r != '\t' })
-	prefix := strings.Repeat("\t", prefixLen)
-	for _, line := range strings.Split(script, "\n") {
-		line = strings.TrimRightFunc(line, unicode.IsSpace)
-		line = strings.TrimPrefix(line, prefix)
-		s.Cmds = append(s.Cmds, line)
-	}
-}
-
-// MarshalJSON marshals shell commands according to s.MarshalAs. It marshals
-// commands to a string by joining s.Cmds with newlines.
-func (s ConfigShellCmds) MarshalJSON() ([]byte, error) {
-	switch s.MarshalAs {
-	case CmdArray:
-		return json.Marshal(s.Cmds)
-	case CmdString:
-		return json.Marshal(s.String())
-	default:
-		panic(fmt.Sprintf("invalid command format: %s", s.MarshalAs))
-	}
-}
-
-// UnmarshalJSON unmarshals shell commands from a string, an array of strings,
-// or null. When the JSON value is a string, it unmarshals into the first index
-// of s.Cmds.
-func (s *ConfigShellCmds) UnmarshalJSON(data []byte) error {
-	if len(data) == 0 || string(data) == "null" {
-		s.MarshalAs = CmdArray
-		s.Cmds = nil
-		return nil
-	}
-
-	switch data[0] {
-	case '"':
-		s.MarshalAs = CmdString
-		s.Cmds = []string{""}
-		return json.Unmarshal(data, &s.Cmds[0])
-
-	case '[':
-		s.MarshalAs = CmdArray
-		return json.Unmarshal(data, &s.Cmds)
-	default:
-		return nil
-	}
-}
-
-// String formats the commands as a single string by joining them with newlines.
-func (s *ConfigShellCmds) String() string {
-	return strings.Join(s.Cmds, "\n")
-}
-
 // findConfigDir is a utility for using the path
 func findConfigDir(path string) (string, error) {
 	debug.Log("findConfigDir: path is %s\n", path)