[vim/vim] runtime(shaderslang): fix b:match_words breaking % on braces (PR #21064)

In `ftplugin/shaderslang.vim`, `b:match_words` puts `{` in the same *open* half as `if`/`for`/`while`/`switch`/`struct`/`class`, and `}` in the same *close* half as `break`.
matchit counts every alternative in a group instead of pairing them with each other, so `for (...) {` counts as two openers and a brace can pair with a `break`.

With `vim --clean -c 'packadd matchit' -c 'edit x.slang'` (in Nvim `nvim --clean x.slang` is enough, it enables matchit by default):

```slang
void f() {                       // % -> line 7   (should be 13)
  switch (mode) {                // % -> line 4   (should be 7)
    case 1:
      break;
    default:
      break;
  }                              // % -> line 1   (should be 2)
  for (int i = 0; i < 4; ++i) {  // % -> line 12  (correct)
    if (i == 2)
      break;
    total += i;
  }
}                                // % -> line 8   (should be 1)
```

With one more keyword line in the mix (e.g. an `if` above the `break` in `case 1`) the surplus stops balancing altogether and `%` on the function's opening brace finds no match at all, leaving the cursor where it is.

### Fix

Drop the brace/keyword group and `\[:\]` — matchit appends `'matchpairs'` by itself.
The preprocessor group is left unchanged: Slang has no `#elifdef`/`#elifndef` (`kDirectives` in `slang-preprocessor.cpp`), so it should not follow `ftplugin/c.vim` there. `b:match_skip` is untouched.

Trade-offs: braces match correctly again, brackets and parens are unaffected, and `%` on `if`/`for`/`while` now goes to the header's closing paren as in C files - but `%` on a `break;` no longer jumps back to the enclosing opener.
Keeping that would need a group of its own (e.g. `^\s*\<\(do\|for\|switch\|while\)\>:^\s*\<break\>`), which in turn leaves `%` dead on any `for`/`while` without a `break` in it.

### Tests

`Test_shaderslang_matchit_switch_break` and `Test_shaderslang_matchit_loop_break` in `src/testdir/test_plugin_matchit.vim` fail before this change and pass after it.

cc @mTvare6 as the file's maintainer.

Prepared with AI assistance (Claude Opus 5) per CONTRIBUTING.md, recorded in the commit's `Co-Authored-By` trailer.

Mainly, I wanted to report this - the patch is a suggestion, written with AI assistance and reviewed as far as my understanding goes. So, please change or replace it as you see fit. If I can help with anything, let me know :)

You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/21064

-- Commit Summary --

  * runtime(shaderslang): fix b:match_words breaking % on braces
  * Merge branch 'vim:master' into shaderslang-match-words

-- File Changes --

    M runtime/ftplugin/shaderslang.vim (9)
    M src/testdir/test_plugin_matchit.vim (18)

-- Patch Links --

https://github.com/vim/vim/pull/21064.patch
https://github.com/vim/vim/pull/21064.diff

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