Bump `generate-github-markdown-css` version for missing vars

[jjspace]

Quick PR to bump dependency version. Fixes #138 Fixes #137

Github has updated it's css with new variable name formats and added a few more that the previous version of generate-github-markdown-css was missing. This was important because the external support added for Github's Alert syntax in other extensions is broken without them. _{I still don't agree with that syntax (see discussion but the extension shouldn't be broken}

@mjbvz I wasn't sure if this would be a minor or patch version bump so I left that up to you.

Testing

• Run npm run build to generate the new versions of the CSS files
• Run the extension with a test markdown file
• Ensure all themes still look good

Edit: I don't have the alert extension installed but I just realized that Markdown All in One now adds support for it and it did have the wrong rendering so that's a way to verify.

[jjspace]

Bump @mjbvz, can you take a quick look at this?