Disable CHANGELOG.json by default

[ecraig12345]

Nothing in beachball reads the CHANGELOG.json file, and it's not that useful to users, while potentially using a lot of space in git history as noted in #978. (It's the only place where change entries with type none are recorded, but this probably has limited benefit; we could also add an option to include those in the markdown under "other changes" if desired.)

In the current version, we can add an option writeChangelogJson which defaults to true. In a major release, we can change the default of this option to false.

We have an option generateChangelog which currently defaults to true. #980 added 'md' and 'json' as possible values for this option, to allow writing only markdown (or only json if someone wants that for some reason).

In a major release, we can consider changing the default for generateChangelog to 'md'.