Send ^L when the terminal clear command is triggered on supported shells

[Tyriar]

Currently we tell xterm.js to clear the viewport when the clear terminal command is triggered. This does not work for the case where a multi-line prompt is used.

[Tyriar]

To verifier:

In bash and zsh set your $PS1 to include a \n character, then run the Terminal: Clear command and make sure your full multi-line prompt is retained.

Also check on a non-supported shell like cmd.exe that it clears as before (multiple lines are not retained).

[EyePulp]

Howdy; running vscode Version 1.23.0-insider on MacOS

I have { "key": "cmd+k", "command": "workbench.action.terminal.clear", "when": "terminalFocus" }, set in my keybindings.json.

cmd+k works as expected in my terminal if I'm at a prompt. However, starting a few weeks ago, if a script is running when cmd+k is pressed, I see a ^L show up, but the term window isn't cleared.

The timing seems related to this change, but I'm not sure. Advice is appreciated if this isn't a bug.
Thanks!

[Tyriar]

@EyePulp thanks for the report, created #48974

[EyePulp]

Thanks @Tyriar