feat(developer): Add support for 14.0 style word breaker spec

**Is your feature request related to a problem? Please describe.**

Per https://github.com/keymanapp/help.keyman.com/pull/175#pullrequestreview-403869829, the word breaker spec has changed for lexical models. We need to update the model editor to support the additional field, or at least the 14.0 format which is an object rather than a string. At the same time, we should consider whether we want Developer 14.0 to support saving 12.0/13.0-format model files -- this has ramifications for future changes!

Old format:

```
wordBreaker: 'default',
```

New format (`joinWordsAt` is optional):

```
wordBreaker: {
use: 'default',
joinWordsAt: ['-'], // join words that contain hyphens
},
```

Start in the model editor's wordBreaker parsing and saving entry points, using the linked word breaker specification change as context. Done means supporting the 14.0 object format, including optional joinWordsAt, and resolving whether 12.0/13.0 model files must remain save-compatible.

Written by the indexing model from the issue text.