String#gsub! Elide MatchData allocation when we know it can't escape

In gsub is used with a string replacement or a map that doesn't
have a default proc, we know for sure no code can cause the MatchData
to escape the `gsub` call.

In such case, we still have to allocate a new MatchData because we
don't know what is the lifetime of the backref, but for any subsequent
match we can re-use the MatchData we allocated ourselves, reducing
allocations significantly.

This partially fixes [Misc #20652], except when a block is used,
and partially reduce the performance impact of
abc0304cb28cb9dcc3476993bc487884c139fd11 / [Bug #17507]

```
compare-ruby: ruby 3.5.0dev (2025-02-24T09:44:57Z master 5cf146399f) +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-02-24T10:58:27Z gsub-elude-match da966636e9) +PRISM [arm64-darwin24]
warming up....

|                 |compare-ruby|built-ruby|
|:----------------|-----------:|---------:|
|escape           |      3.577k|    3.697k|
|                 |           -|     1.03x|
|escape_bin       |      5.869k|    6.743k|
|                 |           -|     1.15x|
|escape_utf8      |      3.448k|    3.738k|
|                 |           -|     1.08x|
|escape_utf8_bin  |      6.361k|    7.267k|
|                 |           -|     1.14x|
```

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>

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