Fix panic when interpolating fields in a record type

[jneem]

Previously, we would panic on {"%{a}": Number}. This makes it an error instead, with an error message about record types not allowing interpolation in fields.

There's another option here, which is to update is_record_type to check for the presence of interpolation, but I opted against it because I thought the error message was worse: since it would see that parsing as a record type would fail, it would fall back to parsing as a record literal and then fail later with error: statically typed field without a definition.

The same choice will come up in #1284