Allow JSONC in .buildifier.json config parsing

[alexeagle]

Summary

• Parse .buildifier.json as JWCC/JSONC using github.com/tailscale/hujson, so configs can include // and /* */ comments and trailing commas.
• Add unit tests covering comment stripping edge cases and end-to-end config loading.

Justification

Editors and many tooling ecosystems treat .json config files as JSONC (VS Code, Cursor, etc.), but buildifier previously used encoding/json directly and rejected any comments. That mismatch forces users to choose between documented/annotated configs and a config that buildifier will actually load.

Using hujson gives spec-compliant JWCC parsing (RFC 8259 JSON plus comments and trailing commas) instead of a hand-rolled comment stripper, which is safer around strings containing // or /*.

Test plan

• bazel test //buildifier/config:config_test
• Verify a .buildifier.json with inline comments loads via --config

[gemini-code-assist]

Code Review

This pull request introduces support for JSON with comments (JSONC) and trailing commas in the configuration parser by integrating the hujson library. It includes the helper implementation, unit tests, and necessary dependency updates. The reviewer suggested wrapping the error returned by json.Unmarshal in config.go to maintain consistency with other error-wrapping patterns in the same function.

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[gemini-code-assist]

For consistency with the other error wrapping in this function (reading config and parsing config), the error returned by json.Unmarshal should also be wrapped to provide clear context if unmarshaling fails.

Suggested change

	if err := json.Unmarshal(data, c); err != nil {
	return err
	}
	if err := json.Unmarshal(data, c); err != nil {
	return fmt.Errorf("unmarshaling config: %w", err)
	}

References

1. When modifying code, ensure that changes are consistent with previous behavior, especially if the previous behavior was intentional.