Upload only changed gist files

[Noshkas]

Short description of what this resolves:

Only changed settings files are included in the Gist update payload when uploading settings, so unchanged files are not re-uploaded and the upload summary reflects the files changed during the upload.

Changes proposed in this pull request:

• Detect changed, new, and locally deleted Gist files before saving the Gist.
• Patch regular uploads with only changed files plus cloudSettings, while keeping new Gist and force-upload behavior unchanged.
• Add a focused GitHubService test for partial Gist updates and explicit deletions.

Fixes: #508

How Has This Been Tested?

• npm run tslint-check && ./node_modules/.bin/tsc -p ./
• npm run compile
• Targeted Mocha check for GitHubService.UpdateChangedFiles with a minimal VS Code runtime stub.

Note: The repository's plain npm test command reaches Mocha after lint/typecheck, but then attempts to load the vscode runtime module outside the VS Code extension host in this local setup.

Screenshots (if appropriate):

N/A

Checklist:

• I have read the contribution guidelines.
• My change requires a change to the documentation and GitHub Wiki.
• I have updated the documentation and Wiki accordingly.