feat(AIP-192): Catch unpaired backticks in comments

[acamadeo]

Part of #1208

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[noahdietz]

Hey @acamadeo thanks for the contribution. I'm a bit swamped and won't be able to take a deep look this week.

In general though, we have avoided linting specific markdown formatting bugs because we don't want ApiLinter to become a markdown parser. The complexity of the proposed logic for just backticks is an example of why. Personally, I think it would be better if there was a separate tool to validate CommonMark syntax in proto comments, aside from ApiLinter.

[acamadeo]

No worries, and that makes sense. I'm happy to withdraw the PR if you determine it ends up adding too much complexity to the codebase.