Optimize `File.extname` for common encodingsHEADmaster

Similar optimizations to the ones performed in GH-15907.

- Skip the expensive multi-byte encoding handling for the common
  encodings that are known to be safe.
- Use `CheckPath` to save on copying the argument and only scan it for
  NULL bytes once.
- Create the return string with rb_enc_str_new instead of rb_str_subseq
  as it's going to be a very small string anyway.

This could be optimized a little bit further by searching for both `.` and `dirsep`
in one pass,

```
compare-ruby: ruby 4.1.0dev (2026-01-19T03:51:30Z master 631bf19b37) +PRISM [arm64-darwin25]
built-ruby: ruby 4.1.0dev (2026-01-20T07:33:42Z master 6fb50434e3) +PRISM [arm64-darwin25]
```

|           |compare-ruby|built-ruby|
|:----------|-----------:|---------:|
|long       |      3.606M|   22.229M|
|           |           -|     6.17x|
|long_name  |      2.254M|   13.416M|
|           |           -|     5.95x|
|short      |     16.488M|   29.969M|
|           |           -|     1.82x|

0 files changed, 0 insertions, 0 deletions