Support self update --output-format=json

[InSyncWithFoo]

Summary

Resolves #14455.

self update --output-format=json outputs the result in JSON. The outputs should look like these:

{"result":"offline"}
{"result":"externally-installed"}
{"result":"multiple-installations","current":"/home/some/bin","other":"/home/some/other/bin"}
{"result":"github-rate-limit-exceeded"}
{"result":"on-latest","version":"0.7.19"}
{"result":"would-update","from":"0.7.18","to":"latest"}
{"result":"would-update","from":"0.7.18","to":"0.7.19"}
{"result":"updated","from":null,"to":"0.7.19","tag":"0.7.19"}
{"result":"updated","from":"0.7.18","to":"0.7.19","tag":"0.7.19"}

Test Plan

Unit tests.

[InSyncWithFoo]

I'm not sure how to test the more important cases (on-latest, would-update, updated). Existing tests don't seem to cover them either.