Previously any expansion closing sequence would've caused the entire
expansion chain to be terminated, fix this by keeping track of active
expansions and running the parser in 'skip' mode.
Fixes#19110.
Previously we were starting the variable expansions before the starting
'$' and calculating the wrong length, this commit makes it so we
calculate the right range for them.
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.
