Shell: Start implementing a POSIX-compliant parser

The parser is still very much a work-in-progress, but it can currently
parse most of the basic bits, the only *completely* unimplemented things
in the parser are:
- heredocs (io_here)
- alias expansion
- arithmetic expansion

There are a whole suite of bugs, and syntax highlighting is unreliable
at best.
For now, this is not attached anywhere, a future commit will enable it
for /bin/sh or a `Shell --posix` invocation.

Userland/Shell/CMakeLists.txt

@@ -12,6 +12,8 @@ set(SOURCES
     Job.cpp
     NodeVisitor.cpp
     Parser.cpp
+    PosixLexer.cpp
+    PosixParser.cpp
     Shell.cpp
 )