Add syntax highlighting of Pkl code

[bioball]

This adds syntax highlighting of Pkl code!

It adds highlighting for:

• Stack frames within error messages
• CLI REPL (highlights as you type, highlights error output)
• Power assertions (coming in Implement power assertions #1384)

This uses the lexer for highlighting. It will highlight strings, numbers, keywords, but doesn't understand how to highlight nodes like types, function params, etc.
The reason for this is because a single line of code by itself may not be grammatically valid.

Sample screenshots:

[stackoverflow]

LGTM!