Add string-replace-fewer-backslashes feature

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.

tests/invocation/features-string-backslashes-off.invoke

@@ -0,0 +1 @@
+--features 'no-string-replace-fewer-backslashes' -C 'string replace -ra "\\\\" "\\\\\\\\" -- "a\b\c"'

tests/invocation/features-string-backslashes-off.out

@@ -0,0 +1 @@
+a\b\c

tests/invocation/features-string-backslashes.invoke

@@ -0,0 +1 @@
+--features 'string-replace-fewer-backslashes' -C 'string replace -ra "\\\\" "\\\\\\\\" -- "a\b\c"'

tests/invocation/features-string-backslashes.out

@@ -0,0 +1 @@
+a\\b\\c

tests/status.out

@@ -6,5 +6,6 @@ test_function
 # Future Feature Flags
 stderr-nocaret	off	3.0	^ no longer redirects stderr
 qmark-noglob	off	3.0	? no longer globs
+string-replace-fewer-backslashes	off	3.1	string replace -r needs fewer backslashes in the replacement
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