Replace uses of the word "accessor" in diagnostics with user-facing terminology

We currently have a bunch of type-checker diagnostics that uses the word "accessor" instead of user-facing words such as "getter", "setter" or "observer". The word "accessor" is actually not used anywhere in user-facing documentation or TSPL.

It would be great to replace uses of "accessor" with words that the user is more likely to be familiar with, as mentioned above.