chore: Support starlark with .sky extension

[honnix]

.sky extension is used by e.g. copybara, and it would be better to get it supported.

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Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: no
• Breaking change (forces users to change their own code or config): yes
• Suggested release notes appear below: yes

• .sky file extension is not exclusive to Starlark (Skylark), so this might be overshooting
• existing .sky Starlark files will get formatted with this change, so that might break existing user code

Test plan

• Manual testing; please provide instructions so we can reproduce: We have been using a patch in production for some time and it's working well.

[aspect-workflows]

Bazel 7 (Test)

2 test targets passed

Targets

//format/test:custom_args_test [k8-fastbuild] 335ms
//format/test:ls-files_test [k8-fastbuild]    706ms

Total test execution time was 1s. 4 tests (66.7%) were fully cached saving 500ms.

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Bazel 8 (Test)

2 test targets passed

Targets

//format/test:custom_args_test [k8-fastbuild] 441ms
//format/test:ls-files_test [k8-fastbuild]    634ms

Total test execution time was 1s. 4 tests (66.7%) were fully cached saving 475ms.

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Bazel 9 (Test)

2 test targets passed

Targets

//format/test:custom_args_test [k8-fastbuild] 277ms
//format/test:ls-files_test [k8-fastbuild]    436ms

Total test execution time was 713ms. 4 tests (66.7%) were fully cached saving 427ms.