Prior to this fix, an escaped character like \x41 (hex for ascii A)
was interpreted the same was as A, so that $\x41 would be the same
as $A. Fix this by inserting an INTERNAL_SEPARATOR before these escapes,
so that we no longer treat it as part of the variable name.
This also affects brackets; don't treat echo $foo\1331\135 the same as
echo $foo[1].
Fixes#7969
Expansion parses slices like "$PATH[1..2]", but so does "set" when assigning
"set PATH[1..2] . .". Commit be06f842a ("Allow to omit indices in index
range expansions") forgot the latter.
This one tests a bunch of separate stuff, so we put it into a few
different files.
The main, new one is "slices.fish", which tests various index expressions.
