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.

Meta/CMake/all_the_debug_macros.cmake

@@ -167,6 +167,7 @@ set(SH_DEBUG ON)
 set(SHELL_JOB_DEBUG ON)
 set(SH_LANGUAGE_SERVER_DEBUG ON)
 set(SHARED_QUEUE_DEBUG ON)
+set(SHELL_POSIX_PARSER_DEBUG ON)
 set(SIGNAL_DEBUG ON)
 set(SLAVEPTY_DEBUG ON)
 set(SMP_DEBUG ON)