meta: add `base` as a type and top level property

[mvo5]

Thanks for helping us make a better Snapcraft!

LP: #1706564

[sergiusens]

Thanks for the contribution. The tests failed because your commit in the email has not signed the CLA, you can either sign it with your @ubuntu.com account or use your @canonical.com email to author the commit.

[mvo5]

The contributor agreement thing is hopefully sorted now.

[elopio]

Thanks for the change @mvo5!
This needs a user test. Ideally, this would call snapcraft with a different base and execute it. We have snaps_tests for this things, but I'm not sure how easy would it be to automate. If it's too complex, we always have manual_tests.md

[sergiusens]

In the branch on snapd where you mention the dependency on this one you also use base as a type. I don't see that reflected here.

[mvo5]

Thanks for the feedback and sorry for the delay, I was on vacation.

I added the "base" to the support types now and also added tests to snaps_tests as suggested. This will validate that the snaps can be build and installed. If you need more tests, please let me know.