Add a new built-in handler to open files with system application

[dhuebner]

What it does

Adds a new Command and an OpenWithService handler to open files using system application.

How to test

• Select a file in navigator. Call "Open With System Editor" command from the command palette.
• ...or, from the Navigator menu. Select "Open With...". Select the new "System Editor" entry and press ENTER.

In both cases the file should be opened with your system default editor.

Follow-ups

This new command can be called to handle vscode.env.openExternal request, see #13535 .

Review checklist

• [ x] As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines