Stabilize extension terminals (vscode.Pseudoterminal)

[Tyriar]

No description provided.

[Tyriar]

This was tested in depth last iteration so to verify just make sure that the API is present as a non-proposed API and doesn't require enableProposedApi in the package.json to use.

Simple example:

const writeEmitter = new vscode.EventEmitter<string>();
const pty: vscode.Pseudoterminal = {
  onDidWrite: writeEmitter.event,
  open: () => writeEmitter.fire('\x1b[31mHello world\x1b[0m'),
  close: () => {}
};
vscode.window.createTerminal({ name: 'My terminal', pty });