[lldb-dap] Implement command directives (#74808)

This adds support for optionally prefixing any command with `?` and/or
`!`.
- `?` prevents the output of a commands to be printed to the console
unless it fails.
- `!` aborts the dap if the command fails.

They come in handy when programmatically running commands on behalf of
the user without wanting them to know unless they fail, or when a
critical setup is required as part of launchCommands and it's better to
abort on failures than to silently skip.