Initial share provider API and UI

[joyceerhl]

For #176316

Introduces a share provider API and a top-level Share... command and icon in the command center toolbar. There's currently only one provider (from the builtin GitHub extension), more to come. Everything is hidden behind an unofficial setting workbench.experimental.share.enabled, demo:

share.mp4

Screenshot:

[joyceerhl]

@bpasero this PR introduces a success/confirmation dialog to support a Share experience, given prior art in Powerpoint/Word, e.g.:

This also addresses a pain point where copy link actions today do not display success/confirmation and can fail silently. I would appreciate your review of the dialog-related changes here please--thank you!

[abhijit-chikane]

I think clicking on share from command palette should directly open that dialog

[joyceerhl]

@abhijit-chikane yes, it does, I just didn't include that in the recording (the recording shows that Share... exists in the palette and that there is a button to access it which does the same thing)

[bpasero]

I find the introduction of a Success severity strange tbh. I think the current values for Severity make sense, but Success is not a severity for me.

[joyceerhl]

@bpasero would it help to rename Success to Confirmation, or to rename Severity to Reason?

[bpasero]

I think severity is used in many contexts, but for dialogs specifically maybe something else should be used. Here is what Electron calls it:

vscode/src/vs/base/parts/sandbox/common/electronTypes.ts

Lines 19 to 25 in adbcbfa

	/**
	* Can be `"none"`, `"info"`, `"error"`, `"question"` or `"warning"`. On Windows,
	* `"question"` displays the same icon as `"info"`, unless you set an icon using
	* the `"icon"` option. On macOS, both `"warning"` and `"error"` display the same
	* warning icon.
	*/
	type?: string;

[joyceerhl]

@bpasero would it make sense for Success to be a subtype of the info type, which is internally treated differently for dialogs just to distinguish what codicon to use?

[bpasero]

@joyceerhl can you talk with SteVen as component owner of custom dialogs? Maybe someone else took that over, not sure, he would know.