API to find the active comment thread

[alexr00]

This would enable extensions to provide keybindings for the active comment thread.

Example: microsoft/vscode-pull-request-github#5261

[alexr00]

vscode/src/vscode-dts/vscode.proposed.activeComment.d.ts

Lines 6 to 22 in d135428

	declare module 'vscode' {
	// @alexr00 https://github.com/microsoft/vscode/issues/204484
	export interface CommentController {
	/**
	* The currently active comment or `undefined`. The active comment is the one
	* that currently has focus or, when none has focus, undefined.
	*/
	// readonly activeComment: Comment | undefined;
	/**
	* The currently active comment thread or `undefined`. The active comment thread is the one
	* that currently has focus or, when none has focus, undefined.
	*/
	readonly activeThread: CommentThread | undefined;
	}
	}