[FR]: Allow .gitattributes to contribute format language patterns

### What is the current behavior?

Formatting relies on hardcoded patterns for languages in `format/private/format.sh`. Users who wish to customize the language patterns, perhaps to register an internal convention for a file pattern must patch this file.

In our case, we use an OWNERS file format based on the [Kubernetes convention](https://www.kubernetes.dev/docs/guide/owners/) and register `OWNERS` as a `YAML` pattern with the following patch:

```
diff --git a/format/private/format.sh b/format/private/format.sh
index 7d4d7fadab..9a68ab848b 100755
--- a/format/private/format.sh
+++ b/format/private/format.sh
@@ -147,7 +147,7 @@
'TSX') patterns=('*.tsx') ;;
'TypeScript') patterns=('*.ts' '*.cts' '*.mts') ;;
'Vue') patterns=('*.vue') ;;
- 'YAML') patterns=('*.yml' '*.yaml' '.clang-format' '.clang-tidy' '.gemrc') ;;
+ 'YAML') patterns=('*.yml' '*.yaml' '.clang-format' '.clang-tidy' '.gemrc' 'OWNERS') ;;
# Note: https://github.com/github-linguist/linguist/blob/559a6426942abcae16b6d6b328147476432bf6cb/lib/linguist/languages.yml#L7767-L7882
# has a giant list of patterns. We arbitrarily choose some "common" ones.
'XML') patterns=('*.xml' '*.xsd') ;;
```

This patch wouldn't be appropriate to upstream, so we're left indefinitely maintaining a patch on top of `rules_lint` releases.

### Describe the feature

Users with the above use case are likely already managing these file associations in `.gitattributes` for GitHub syntax highlighting. In our case, for the OWNERS example, we have the folllowing in our `.gitattributes` file:

```
OWNERS linguist-language=YAML
```

The feature request would be to update `format/private/format.sh` to query `.gitattributes` for language patterns in `ls-files` to avoid patching the script.

The format script already queries `.gitattributes` for `rules-lint-ignored`, so this change would be in the spirit of the existing implementation and would also honor the `disable_git_attribute_checks` flag.

I propose that we support two conventions for git attributes:
1. A new `rules-lint-language` attribute for users who wish to be explicit
2. Repurpose the existing `linguist-language` to automatically opt-in GitHub language mappings

Start in format/private/format.sh, especially the existing .gitattributes query for rules-lint-ignored and the handling of disable_git_attribute_checks. Trace how language patterns are selected from ls-files, then determine how rules-lint-language and linguist-language should contribute mappings. Done means user-defined patterns such as OWNERS can be formatted without patching the script while the existing disable flag is honored.

Written by the indexing model from the issue text.