Xontrib: Allow using cocounut in rc files and @(...) syntax

I don't know if this would be possible but it would be really great if we could use coconut code in xonsh rc files, python-mode syntax within subprocess mode, and functions.

Example: `rg -n "TODO" @( Path(".").rglob("*.py") |> filter$(p => not p.match("tests/**")) |> filter$(p => p.stat().st_mtime > time.time() - 86400) |> map$(.as_posix()) |> list`