Make completion/symbol item kinds themable

[BoomRaccoon]

So the issue #22206 describes a part of my feature request

But in contrast, all I want is a JSON-key to specify one of the default icons vscode uses for objects, methods and properties
An additional JSON-key to make it easier to differentiate between user snippets and default ones would be for the background-color of user snippets (all user snippets of one language).

[vscodebot]

(Experimental duplicate detection)
Thanks for submitting this issue. Please also check if it is already covered by an existing one, like:

• Allow extending snippets or including snippets in snippet. (#73702)

[vscodebot]

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