[Feature] Add html anchor support

[igor-tsiglyar]

Description

It seems that currently only links to markdown headers are processed, unlike https://github.com/DavidAnson/markdownlint/blob/main/doc/md051.md which also supports links to html anchors.

Describe the solution you'd like

Would be great to have the same processing logic as md051 so that this plugin is usable with anchors.

Describe alternatives you've considered

Current workaround is using https://github.com/tcort/markdown-link-check with the following config:

{
    "ignorePatterns": [
      {
        "pattern": "^#"
      }
    ]
}

This utility also does not work well with anchors so we can ignore all the links to them while links to files are being verified just fine.

[theoludwig]

Hello! @igor-tsiglyar 👋

Thanks for opening this feature request.

It seems that currently only links to markdown headers are processed

Yes, this is the case since the v2 of this custom rule (related: #2), before it was only checking if the files exists and ignoring everything anchors related.
If I remember correctly, when this have been added, it was the same behavior/implementation of markdownlint, and since then, the implementation changed to add more cases other than only headings anchors.

Would be great to have the same processing logic as md051 so that this plugin is usable with anchors.

Definitely, I agree!
Would be much appreciated if someone could open a PR to implement this feature.
The implementation inside markdownlint is here: https://github.com/DavidAnson/markdownlint/blob/main/lib/md051.js and we should certainly take inspiration from there.

A PR for a failing test case would already be a good contribution. 😄
I refactored the tests so it's easier to add new test cases.
Also the README have been updated to describe the current features and limitations of the rule.