fix(common): legacy .kpj schema

Fixes #9140.
Fixes #9148.

Keyman Developer 9.0 .kpj files included a lot of additional state
metadata. We need a schema which validates these files, as they are
otherwise valid to load (we'll never save them any more). Rather than
add all the extra metadata to what is otherwise a fairly clean schema,
we'll provide a legacy .schema.json.

In the future, we may be able to merge these schemas, as we move towards
the .kpj 2.0 format which doesn't list files. Ideally, all three formats
(Keyman Developer 9.0 schema, call it legacy, 1.0 schema for Keyman
Developer 10.0+ which has Options and Files listed, 2.0 schema for
capturing just project settings for a folder) will be supported by a
single schema file.