Make fallthrough explicit in tok_parse.c

[r-barnes]

In response to #325. Allows -Wimplicit-fallthrough to pass.

[earwig]

Indeed, we can see this failed in the windows build: https://ci.appveyor.com/project/earwig/mwparserfromhell/builds/50247115/job/viq4v2m66hgv8878

Let’s duplicate the lines as suggested. I don’t think we can move to C23 yet.

[r-barnes]

@lahwaacz - I've duplicated the lines.

[coveralls]

coverage: 99.201% (-0.002%) from 99.203% when pulling 8c23031f10a329ca80082c30bbf50d07030ee3e7 on r-barnes:patch-1 into 4e73af2fabbbc13915a44f06c283c8bd53558ffa on earwig:main.

[earwig]

Thank you!

[r-barnes]

Thank you!