This disables an extra round of escaping in the `string replace -r`
replacement string.
Currently, to add a backslash to an a or b (to "escape" it):
string replace -ra '([ab])' '\\\\\\\$1' a
7 backslashes!
This removes one of the layers, so now 3 or 4 works (each one escaped
for the single-quotes, so pcre receives two, which it reads as one literal):
string replace -ra '([ab])' '\\\\$1' a
This is backwards-incompatible as replacement strings will change
meaning, so we put it behind a feature flag.
The name is kinda crappy, though.
Fixes#5474.
