fix!: Prevent calling set on the return value of 'computed()'

[Nokel81]

BREAKING CHANGE: 'set()' method now only exists if a 'set()' is provided

Code change checklist

• [x] Added/updated unit tests
• [x] Updated /docs. For new functionality, at least API.md should be updated
• [x] Verified that there is no significant performance drop (yarn mobx test:performance)

resolves https://github.com/mobxjs/mobx/issues/3767