Move local editor settings to a sidecar file

[bodhibuurstede-sys]

Addresses #1983.

Summary

• Persist local editor UI settings in per-project sidecars under .gdevelop/<project-name>.editor-settings.json instead of writing them into the project JSON.
• Restore sidecar settings when opening local projects, while keeping existing embedded settings working when no sidecar exists.
• Remove stale sidecar settings when a project no longer has editor settings to persist.
• Keep sidecars distinct when multiple project files live in the same folder.
• Add coverage for sidecar paths plus extracting/applying layout, external layout, and custom object variant editor settings.
• Make the pre-built libGD.js downloader try matching master/commit/<hash> builds before master/latest, so detached PR CI does not pick an incompatible generated file.

Notes

• The sidecars are intentionally local/user-specific, so teams can gitignore .gdevelop/ to avoid collaboration conflicts.
• I kept this scoped to the local file storage path where the Git conflict problem happens.
• Missing sidecar files are ignored via ENOENT, so normal local projects do not emit a warning before one has been created.

Verification

• Semaphore Fast tests (not building GDevelop.js - can have false negatives): passed on the latest commit.
• Semaphore Flow: passed with No errors! on the earlier run before the CI-only fallback update.
• Semaphore newIDE auto-formatting and newIDE linting: passed on the latest commit.
• Not run locally: this Codex environment does not have the repository dependency setup needed for the full GDevelop test suite.

[bodhibuurstede-sys]

Hi @4ian, the Semaphore PR check is green now after the libGD.js fallback update. This should be ready for review when you have a chance; happy to adjust anything.