fix(core): refine BetterOffPhrase contexts

[cfcryptotrader-rgb]

Summary

• Fixes False negative: "better of" is not flagged as a typo for "better off" #2863.
• Narrows BetterOffPhrase so it still catches clear better of → better off adjective/adverbial contexts from the issue.
• Adds the issue's true-positive examples and false-positive examples as Weir tests.

Duplicate / spam checks

• Issue False negative: "better of" is not flagged as a typo for "better off" #2863 is open, unassigned, and has no comments/reservation.
• Checked active PRs for 2863, better of, and better off; no duplicate found.
• Checked recent merged PRs with the same terms; no duplicate found.

Verification

• rustup run stable cargo fmt --check
• rustup run stable cargo test -p harper-core run_tests_for_better_off_phrase --lib
• rustup run stable cargo test -p harper-core --lib (5307 passed, 264 ignored)
• git diff --check

[hippietrail]

I'm not good at more advanced Weir but if all those tests are passing it's probably ready. I did point out that you can use allows for the negative tests.

[hippietrail]

Oh you can use allows followed by just one copy of the sentence.
It's a bit newer than using test followed by two copies, so there are still linters in the repo using it.