flag march and may when used as month and not verb

This follows a comment of @mrfragger on an issue I opened

> march is a tough one...as it's hard to tell if it's a verb...so should be excluded from checking. Same problem I had for bible names Mark... mark as a verb.
>
> My thinking is that ones like "march" and "may" will probably need dedicated lints involving heuristics based on the words in their context.

_Originally posted by @hippietrail in [#458](https://github.com/Automattic/harper/issues/458#issuecomment-2620831462)_

I hesitated in:
- reopening #458
- adding something to #461, but then the scope would have been different
- opening a new issue or completing

I chose the last option

Start by reading the linked discussion in #458 and comparing the scope of #461; the issue names no files, tests, or entry points. Identify the existing lint structure and context-handling tests before deciding how month-versus-verb cases should be recognized. Done means march and may are flagged only in month usage, with coverage for the relevant contexts.

Written by the indexing model from the issue text.