chore: Add changelog and bump version

Author: tgodzik

changelogs/3.7.0-RC4.md

@@ -0,0 +1,15 @@
+# Backported changes
+
+- Backport "Upgrade to Scala.js 1.19.0." to 3.7 [#23035](https://github.com/scala/scala3/pull/23035)
+
+
+# Contributors
+
+Thank you to all the contributors who made this release possible 🎉
+
+According to `git shortlog -sn --no-merges 3.7.0-RC3..3.7.0-RC4` these are:
+
+```
+     2  Sébastien Doeraene
+     1  Tomasz Godzik
+```

project/Build.scala

@@ -123,7 +123,7 @@ object Build {
    *  During release candidate cycle incremented by the release officer before publishing a subsequent RC version;
    *  During final, stable release is set exactly to `developedVersion`.
   */
-  val baseVersion = s"$developedVersion-RC3"
+  val baseVersion = s"$developedVersion-RC4"
 
   /** The version of TASTY that should be emitted, checked in runtime test
    *  For defails on how TASTY version should be set see related discussions: