This commit also removes the ExecutionInputType and directly uses
RefPtr<Shell> instead, since nothing else is needed for execution
purposes, and also makes the shell refuse to evaluate commands with
any sort of syntax error.
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