Export Action output as JSON

[wtrocki]

Current process

Current Action prints values to console:

golang.org/x/exp/apidiff
  Incompatible changes:
  - Report.Compatible: removed
  - Report.Incompatible: removed
  Compatible changes:
  - Change: added
  - Report.Changes: added

and fails if incompatible changes are present.

Expected result

1. Action should return values as action.output.
2. Action should return values as json

#28 does first step to introduce some level of output.
Users can put output value as comments on PRs or even parse it.

[joelanford]

I had a similar thought here: #21 (comment)

I'll get to this eventually, but I'm also happy to accept a contribution as well!

[wtrocki]

I think realistically JSON output should be:

{
"message": (actual output)
"semver_type": (added in  https://github.com/joelanford/go-apidiff/pull/28)
} 

[wtrocki]

Implemented in #32. @joelanford this is up to your consideration.
IMHO parsing of the Incompatible changes: can happen outside the action source code as it is subject to change and might affect long-term support of the action.