Adopt EnvironmentVariableCollection.description in git extension

[Tyriar]

There's a new API (#171108) being finalized this month that allows extensions to explain why it's changing environment variables. Adopting this will show additional information when hovering a terminal tab

API:

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

Lines 27 to 33 in e6936ce

	export interface EnvironmentVariableCollection extends Iterable<[variable: string, mutator: EnvironmentVariableMutator]> {
	/**
	* A description for the environment variable collection, this will be used to describe the
	* changes in the UI.
	*/
	description: string | MarkdownString | undefined;
	}

Current:

After description:

cc @karrtikr

[lszomoru]

@Tyriar, will you create a test plan item or add verification steps for this? Thanks!

[Tyriar]

To verify:

1. For each combination of settings git.terminalGitEditor and git.terminalAuthentication
2. Open a terminal
3. Hover the tab
4. Verify the description next to Git makes sense