Reuse lint/parser infra for invalid keys error printing

[samhh]

See #179.

[samhh]

This looks easier said than done.

We only parse and keep AST annotations/offsets for ICU messages themselves. The JSON is parsed as a quick conversion roughly Text -> Map.

The easiest route may be shifting this check into the JSON compiler, but then the JSON compiler needs to know about all the code compilers, and conditionally call them all depending on the backend value. This could get messy.