This does not work perfectly (just like every other shell...), if the
running program handles the signal (SIGINT in this case) and quits
cleanly, the shell cannot detect the interruption.
This is the case with our `sleep(1)`.
This commit fixes job control by putting children in their own process
group, and proxying TTY signals to active jobs.
This also cleans up the code around builtin_disown a bit to use
the newer job interfaces.
This commit also completely reworks the execution, highlighting and
completion model to work with the new AST.
New additions:
- $(...) stdout captures
- fd>&fd redirections
- fd>&- redirections (close fd)
- read-write redirections (<> path)
- completely event-based execution
- the weird idea of allowing the user to redirect the shell's own fds
- variables in strings
- local variables
- minimal list support
- adding hyperlinks to all paths that exist
