Adds a new GH action to update the CHANGELOG

[JoeRobich]

Log from running against prerelease

[09:07:14] Loaded external module: ts-node/register
[09:07:16] Using gulpfile ~/Source/omnisharp-vscode/gulpfile.ts
[09:07:16] Starting 'updateChangelog'...
Determining version from CHANGELOG
Adding PRs for 2.93 to CHANGELOG
Using tag v2.92.18-prerelease for previous version 2.92
PRs [#8646, #8639] already in the CHANGELOG
Generating PR list from v2.92.18-prerelease to HEAD
Executing: roslyn-tools pr-finder -s "v2.92.18-prerelease" -e "HEAD" --format o#
PR #8639 is already present in the CHANGELOG
Adding new PR to CHANGELOG: * Add RESX file nesting pattern for Designer.cs files (PR: [#8644](https://github.com/dotnet/vscode-csharp/pull/8644))
Adding new PR to CHANGELOG: * use '.NET' instead of '.NET Core' (PR: [#8638](https://github.com/dotnet/vscode-csharp/pull/8638))
Writing 2 new PRs to the CHANGELOG
[09:07:16] Finished 'updateChangelog' after 232 ms

CHANGELOG diff

  # 2.93.x
+ * Add RESX file nesting pattern for Designer.cs files (PR: [#8644](https://github.com/dotnet/vscode-csharp/pull/8644))
+ * use '.NET' instead of '.NET Core' (PR: [#8638](https://github.com/dotnet/vscode-csharp/pull/8638))
  * Bump Roslyn to 5.0.0-2.25472.11 (PR: [#8646](https://github.com/dotnet/vscode-csharp/pull/8646))
    * Fix handling edits in types nested in reloadable types(PR: [#80360](https://github.com/dotnet/roslyn/pull/80360))