make fish compatible with pcre2 10.21

pcre2_substitute() now sets the output buffer length to PCRE2_UNSET (~0)
if the output buffer is determined to be too small. This change keeps
track of the buffer size separately where pcre2 can't touch it.

A better fix would be to let pcre2 tell fish what size buffer it needs.
This can be done with PCRE2_SUBSTITUTE_OVERFLOW_LENGTH, but this
requires pcre2 10.21 or later (released January 12), which may be too
new to introduce as a dependency at this point.

Fixes #2743

tests/string.out

@@ -41,6 +41,7 @@ spaces_to_underscores
 right left $
 put a
 here
+aabb
 invalid expression error
 invalid argument error
 missing argument returns 0