feat(developer): use 'keyman.kpj' as fixed filename for projects

## Project Filenames

Subnote about project filenames: currently, these are tied to the parent folder filename. This is not ideal, because:
1. it allows for multiple .kpj files in one folder
2. it is easy to desync if either folder or .kpj are renamed
3. makes logic more complicated around testing for .kpj presence

**Proposal:** What if project filenames were fixed? We could go with `keyman.kpj` as a fixed filename for v2.0 projects?

_Originally posted by @mcdurdin in https://github.com/keymanapp/keyman/issues/2761#issuecomment-1813696014_

This will impact:
* Keyman Developer IDE
* kmc
* keyboard and model repositories
* documentation

Note: search for #10113 in source to find TODO references for this.