REST API: On comment create, return an error if the `type` property is set to...

REST API: On comment create, return an error if the `type` property is set to anything other than `comment`.

Of the default comment_types, only comments are expected to be created via the REST API endpoint. Comments do not have registered types the way that Posts do, so we do not have a method to accurately check permissions for arbitrary comment types.

Props dd32, boonebgorges, rachelbaker.
Fixes #38820.
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git-svn-id: https://core.svn.wordpress.org/trunk@39230 1a063a9b-81f0-0310-95a4-ce76da25c4cd