Add trailing whitespace validator

[reidbaker]

• Add rule for trailing whitespace
• Update the skill for adding new validators

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request introduces the TrailingWhitespaceRule to the Dart skills linter, which identifies trailing whitespace in SKILL.md files while permitting exactly two spaces for Markdown hard line breaks. The PR also updates the project's documentation, including a new step-by-step guide for implementing rules, and enables the rule in the local configuration. Review feedback highlights the need to optimize RegExp instantiation for performance, improve pluralization in error messages, and resolve an inconsistency between the provided documentation and the existing rule registration architecture.