Previously this was only an error when useDefineForClassFields: true,
but now it's an error for all code. This is a rare mistake to make, and
usually only occurs in code written before `readonly` was available.

Codefix to come in subsequent commits.

1. Add add-all test
2. Add codefix that delegates to get-set accessor refactor.

Needs massive amounts of cleanup and deduplication.

1. Fix lint.
2. Make code easier to read.
3. Turns some asserts into bails instead.