CI: Add changed-files GitHub Action

[Repiteo]

Replaces our existing shell solution for determining changed files with a dedicated action: changed-files. While this process takes slightly longer than the shell script, it appears to have a much higher consistency with actually gathering the files. Most notably: this enables static checks on forks!

[AThousandShips]

Let's just add a dummy change temporarily to ensure this works correctly

[Repiteo]

Dummy changes with a new file, renamed file, and modified file. The modified file was one of the few files in the entire repo with whitespace/"unsafe" characters in the name, so it doubles as a test to ensure those are processed correctly

[Repiteo]

The results in a pull request context are entirely as expected:

CHANGED_FILES: "dummy.txt" ".github/workflows/static_checks.yml" "modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Tests/TestData/GeneratedSources/AbstractGenericNode(Of T)_ScriptProperties.generated.cs" "dummy-rename.clangd"

The local push changes are technically working as expected, but reflect a potential inherent limitation where only the most recent commit is compared against (.github/workflows/static_checks.yml excluded):

CHANGED_FILES: "dummy.txt" "modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Tests/TestData/GeneratedSources/AbstractGenericNode(Of T)_ScriptProperties.generated.cs" "dummy-rename.clangd"

Having said that, there's an obscene number of inputs this action can accept, so it might be feasible to let it compare against the main branch of the repo instead. I'll do some testing today to verify

[AThousandShips]

Should compare against the base branch or things will break badly when 4.6 branches off etc.

[Repiteo]

Finally got back to this & to a point that I'm totally satisfied with the output. I've accepted pushes falling back to the previous commit as a limitation of git itself, and will drop the idea of attempting upstream hacks (for now…)

While there's absolutely potential for more specialized output, I think this is a reasonable starting point. Future PRs can build off this action now that it exists, meaning conditionally enabling/disabling tests based on what's been changed (docs-only not running builds, android-only building just android, etc)

[Repiteo]

Updated to have static_checks output the changed-files variable, meaning the clang-tidy check in linux_builds can safely and expediently reuse the exact same list of changed files @Ivorforce

[Ivorforce]

Using an action for this sounds good with me if we've had issues with reliability of the shell script before.

I don't know what a "treeless clone" is but the runtime/speed of the action seems fine.
I'm impartial on style, so maybe needs a more opinionated review.

[Ivorforce]

This change will make it impossible to call this workflow without providing it changed files to check. Maybe we should make it not required and act accordingly (skip the clang-tidy check)?

[Repiteo]

It should be possible to call via workflow_dispatch, with the input variable falling back to an empty string. But as an added precaution, I'll see if I can skip that step on dispatch entirely

[Repiteo]

Update: I verified on my local repository that this is indeed how it works. Given the workflow_dispatch exception might be tricker than expected, I'll just leave this as-is