Add variants of 'check out' to word list

What does this MR do?

Adds a (very rough) stub to differentiate between checkout (a Git command) and "check out" (a phrasal verb). @pedroms and I discovered an instance of "checkout" (not wrapped in code tags) in the UI, and we realized we had no guidance.

• Microsoft: OK to use as a verb phrase, as in check in code or check in changes. Don't use as a noun. (https://docs.microsoft.com/en-us/style-guide/a-z-word-list-term-collections/c/check-in)
• Google: no guidance

I bashed out just enough of a MVC to make clear what was needed; I fully expect @sselhorn will polish it. She may want to add checkin vs check in while we're at it, though I'm not as concerned about it. It doesn't collide with a Git term.

Related issues

Related to #346053 (closed) where we're working on UI message changes for %14.6.

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