Add warnings for split inline code

What does this MR do?

As requested due to the related MR, this is a minimal test for broken inline code spans. It has false positives and does not catch cases where there are multiple inline code spans in a single line, because it's just looking for lines where there is a single backtick (`) which suggests that the closing backtick is missing.

Known false positives:

• https://gitlab.com/gitlab-org/gitlab/-/blob/bc0a1f7b48324a35cba9d63ba3e40944d3481795/doc/development/fe_guide/style/vue.md#L206-216
• https://gitlab.com/gitlab-org/gitlab/-/blob/bc0a1f7b48324a35cba9d63ba3e40944d3481795/doc/development/testing_guide/frontend_testing.md#L999-1003 (and others of a similar style on that page)
• https://gitlab.com/gitlab-org/gitlab/-/blob/bc0a1f7b48324a35cba9d63ba3e40944d3481795/doc/user/project/remote_development/connect_machine.md#L108 (<kbd>`</kbd>)

Related issues

• Related to !148705 (merged)

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