Fixes three false positives when "A" or "B" are school test grades

[hippietrail]

Issues

Fixes #3715

Description

There were two false positives when "A" was a school test grade and one where "B" was a school test grade.

The AnA linter now aborts if an "A" is uppercase and either:

• preceded by "an" or "a"
• Followed by - then another capital letter between "B" and "F"

The ExpandMemoryShorthands linter now aborts if "b" is followed by "+" or "-"

How Has This Been Tested?

cargo test 3715 then full 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.