migrate @storybook/addon-mdx-gfm to strict TS

[1234tgk]

Issue #22176

What I did

Adjust the code so that the package allows strict typing.

How to test

Run yarn check on @storybook/addon-mdx-gfm directory.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[kasperpeulen]

LGTM and thanks!

[jonniebigodes]

@1234tgk, one thing left to address as part of this initiative and the amazing work you did by helping us convert Storybook's packages to allow stricter TypeScript support. If you're ok with it, can you message me on our Discord server ( same username) so that I can follow up with you?

Hope you have a fantastic day.

Stay safe

[1234tgk]

@jonniebigodes Hello! Which discord channel should I message you from? I've only send messages at wg-typescript channel previously.

[jonniebigodes]

If you're ok with it, you can message me directly, or if you're not comfortable with it, drop a message in the wg-typescript channel and tag me and I'll follow up with you.

[1234tgk]

@jonniebigodes Just sent you a friend request instead since I couldn't send a DM to you (Discord blocked me from doing so)

[jonniebigodes]

@1234tgk no worries there. I just saw the notification and will follow up with you.