feat: Add Markdown languages

[nzakas]

This pull request adds two markdown languages to the plugin:

• commonmark - CommonMark syntax
• gfm - GitHub-Flavored Markdown syntax

It also adds several rules that apply to Markdown content. These are all intended to find problems and not to format the document. I've included documentation for each rule.

I changed the recommended config to be for linting Markdown content, not using the processor. There is a new processor config that implements the old recommended config.

Note on types: Because the @types/eslint package defines types specific to ESTree, I can't really add more types into the rules until we have a more generic type definition for rules. I use RuleModule to ensure that the plugin itself will type check correctly, but that definition doesn't allow visitor methods for non-ESTree values.

Refs #160

[nzakas]

Added a new no-invalid-label-ref rule to account for the cases mentioned in https://github.com/eslint/markdown/pull/268#discussion_r1715472241

[nzakas]

I updated no-invalid-label-refs so that it highlights the brackets rather than the label. I think that makes more sense because it's the brackets that are the problem.

[fasttime]

Just the above note, then LGTM.

[nzakas]

Thanks! Yeah Markdownlint has some other options for these rules that allow for more edge cases, but I wanted to start simple. We can always add more options to the rules as we go.