Bash eats the backslash in this format (similarly for W\ORD etc.).
Dr.POSIX doesn't specify this anywhere, but it's used all over the
place, so let's support it.
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.
