API: Contributable icons

[joaomoreno]

Currently icons from extensions seem to come in via:

• ThemableDecorationAttachmentRenderOptions.contentIconPath: Uri
• ThemableDecorationRenderOptions.gutterIconPath: Uri
• DecorationRenderOptions.gutterIconPath: Uri
• Status bar $(octicon) style icons.

The first three are extensible at will. The status bar icons are currently restricted to Octicons.

It would be awesome to have a contribution story here. This could even solve #4201, if we move the completion item icons to this mechanism, by having the default icons be contributed in the same way.

Proposal

Introduce a contributes.icons section:

"icons": [
  {
    "name": "foo",
    "path": "path/to/svg/or/png",
    "dark": {
      "path": "optional/path/to/dark/svg/or/png"
    }
  }
]

[jrieken]

close this as a dupe #10455. We should explore to use icon themes for this. They already define a symbolic-name for icons, like octicons, which then can reference is strings etc