Add anchor link support within detection / jumping / completions

[SidOfc]

This is a proposal to add support to detect these types of links:
Currently, some work can be found in: https://github.com/SidOfc/mkdx/tree/patch/fragment-detection

<a id="top-of-page"></a>


MD: Go to [top](#top-of-page)
HTML: Go to <a href="#top-of-page"></a>

On Github, it would be perfectly valid to go to https://github.com/[USER]/[REPO]#top-of-page but mkdx does not recognize these links as valid fragments. One thing to note with Github and manually inserting a fragment identifier is that the indexes of other (automatically generated) headings do not change. For example:

# This is a header

With some text

## This is a header

While the second header is a H2, the generated fragment will be "#this-is-a-header".
But since this is the second occurrence of this header, a "-[N+1]" suffix is appended.

When adding a fragment identifier like so:

# This is a header

With some text

<a id="this-is-a-header"></a>

## This is a header

This time there is an extra anchor using an "id" value of a header that already exists.
Instead of handling that scenario, GH seems to ignore it. The second occurrence of this header remains the same as in the previous example ("#this-is-a-header-1") and the "id" anchor is simply ignored all together. The only time one can jump to an <a id="..."> or <a name="..."> anchor is when no existing anchor for it exists.