types: fix import

[korki43]

This fixes the import of types from markdown-it.

@types/markdown-it doesn't provide default exports, so I got an error saying something like: Module '"/path/to/@types/markdown-it/index"' can only be default-imported using the 'esModuleInterop' flag. (And yes, I had esModuleInterop activated in my tsconfig)

I fixed it by importing only PluginWithOptions. I don't really understand why this works (and didn't before) but it works so I think it's ok. It should work on all TypeScript build targets.

[coveralls]

Pull Request Test Coverage Report for Build 91

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 98.99%

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Totals
Change from base Build 89:	0.0%
Covered Lines:	53
Relevant Lines:	53

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💛 - Coveralls