feat: RelativePronoun Pattern

[hippietrail]

Issues

N/A

Description

I split this out of my #3611 PR as a separate, clean, tiny PR of its own.

It makes it easy to use relative pronouns in Exprs without adding a new annotation to dictionary.dict since there's only 5 words in the class, but helps when working on a linter when the author only thinks for "that" and "which" and maybe "who" but forgets the other two.

No tests as of yet as the Patterns I base it on didn't have any. If needed where should I put them?

I thought there might be linters that could be upgraded to use this but my quick look at linters using some of these words made it seem like the current ones didn't need the full set - but I may very well have overlooked something.

How Has This Been Tested?

cargo test

AI Disclosure

• I am a human and didn't use any AI.
• I used LLM features of my editor, but not an agent.
• I used an AI agent interactively.
• I am an agent or I got an agent to do the work autonomously.

If Your PR Implements or Enhances a Linter

• I made up the sentences in the unit tests.
• The sentences in the unit tests were generated by an AI.
• I'm using examples from the bug report / feature request.
• I collected real-world sentences for the unit tests.

Checklist

• I have performed a self-review of my own code
• I have added tests to cover my changes
• I have considered splitting this into smaller pull requests.