Add FileDecoration backgroundColor support and tree row rendering

[egorovsa]

Why

This PR implements backgroundColor support for file decorations, addressing my own feature request in #239308.
This issue also related to this feature #31308.

Today vscode.FileDecoration supports text color (color) but not background color.
This change enables extensions to highlight decorated items more clearly by applying a background color in addition to foreground color.

What

• Added backgroundColor?: ThemeColor to vscode.FileDecoration API surface:
  • src/vscode-dts/vscode.d.ts
  • src/vscode-dts/vscode.proposed.codiconDecoration.d.ts
  • ext host types/protocol wiring (extHostTypes, extHost.protocol, extHostDecorations, mainThreadDecorations)
• Extended decoration data model/service:
  • IDecorationData.backgroundColor
  • background-aware decoration key generation
  • CSS rule generation/removal for background color classes
  • IDecoration.bgColorClassName handling
• Updated label rendering pipeline to apply both:
  • foreground/text color class
  • optional background color class
• Implemented stable tree-row background rendering:
  • added dedicated .monaco-tl-decoration-container layer in tree row template
  • applied background class to that dedicated layer
  • ensured class cleanup on re-render/dispose to avoid virtualization artifacts while scrolling
  • added pointer-events: none to decoration layer to avoid interaction regressions
• Added tests covering optional background class behavior in decorations service.

Notes

This PR is a direct implementation of the feature request opened by me in #239308.

After the merge

This feature will allow me to update my extension and provide users with the ability to colorize files like this:

[egorovsa]

@microsoft-github-policy-service agree

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@joaomoreno

Matched files:

• src/vs/base/browser/ui/tree/abstractTree.ts
• src/vs/base/browser/ui/tree/media/tree.css

@benibenj

Matched files:

• src/vs/base/browser/ui/tree/abstractTree.ts
• src/vs/base/browser/ui/tree/media/tree.css