Shell: Switch to a new parser and AST

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

Shell/CMakeLists.txt

@@ -1,4 +1,6 @@
 set(SOURCES
+    AST.cpp
+    Builtin.cpp
     main.cpp
     Parser.cpp
     Shell.cpp