feat: Flag decades with apostrophe in the wrong place

Currently both incorrect and correct usages are flagged with `Did you mean to spell “s” this way?`, which is wrong for other reasons:

What it should do is flag and suggest:

- 80's → '80s
- The 70's → The '70s
- The 1990's → The 1990s

Start by reproducing the decade-apostrophe cases from the issue and tracing the grammar rule that produces the current “s” suggestion. Done means the checker flags 80's, The 70's, and The 1990's with the respective suggestions '80s, The '70s, and The 1990s.

Written by the indexing model from the issue text.