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
