[themes] Add product icon themes

[aeschli]

Fixes #79130

• productIconThemes definition files, with font specification and icon definitions (name -> font character)
• JSON schema for product icon themes associated to -product-icon-theme.json
• New contribution point productIconThemes
• new setting workbench.productIconTheme
• Test Product Icons example in vscode-colorize-tests
• icon registry with all themable icons
• allow to contribute to icon registry from icon contribution point --- Extracted as Allow to contribute to icon registry from icon contribution point #114942
• update svg references to use codicons Cleanup remaining svg icons #86708
• product icon theme selection command
• name icons ids like we did it with color ids (id describes icon location glyphMargin.collapedRange, not the chosen representation (chevron-right). However we want to keep chevron-right as these can be accessed in Markdown or labels using `$(chevron-right)
• update code references to unicode characters (git decorations, debug breakpoints, etc.)
• allow codicons to be used in webviews
• have helper wdget that makes it easy to host an image
• finalize API Finalize product icon theme contributions #113828