fix(core): flag 'less <adjective> that' as a typo for 'than' (#3736)

[hammadmunir959]

Issues

Fixes #3736

Description

Enhances the ThatThan linter in harper-core to catch false negatives when that is used as a typo for than in multi-word comparative phrases formed as less <adjective> that (e.g. "way less common that mp3" -> "way less common than mp3").
Guards against false positives by excluding clause-starting words (is, was, would, had, it, they, those, users, etc.) following that.

Demo

N/A (Core linting rule covered by unit tests).

How Has This Been Tested?

Added unit tests in harper-core/src/linting/that_than.rs (fix_way_less_common_that, fix_less_frequent_that). Verified zero regression against existing ThatThan tests and full test suite.

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.