feat(scratch-def): add 'release' to schema properties for autocomplete

[klewis-sfdc]

What does this PR do?

-- Adds release property to properties section of project-scratch-def.schema.json so that Visual Studio Code will autocomplete and validate that property. --

A community contribution from @douglascayers. See PR #46 for more detail.

What issues does this PR fix or reference?

#, @@

Functionality Before

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Functionality After

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[douglascayers]

Thanks @klewis-sfdc -- curious though, why can't #46 be merged on its own? Why is #52 necessary?

[randi274]

@douglascayers for security reasons our builds don't run on community contributed PRs, so only PRs from authorized contributors create automated builds for tests. Unfortunately we don't distinguish between inside-Salesforce-contributors and outside-Salesforce-contributors, so it does mean an extra step for us.

[douglascayers]

Thanks :)