Add hover text

[bkoropoff]

If the link text is not literally the full identifier, show the full identifier on hover.

This will be handy for the way I intend to handle mkdocstrings/mkdocstrings#269

[oprypin]

Hmm so this change is good for mkdocstrings usage but I think generally this effect can be undesirable. Consider the main example:

https://github.com/mkdocstrings/autorefs#usage

Or um
if I write

Link to [Hello, World!][hello-world]

I don't want to have the popup text "hello-world", what's the use of it?

So I think it should be mkdocstrings itself that somehow indicates that a hover title should be used.

[bkoropoff]

@oprypin I changed it to use a different reference type. I noticed mkdocstrings-identifier seemed to be unused, so I removed it from the regex as well.

[oprypin]

Thanks.
Hmm I don't understand something.
If currently mkdocstrings produces only "required references", shouldn't the new thing be "required references with title"? Instead you seem to be adding "optional references with title".

[bkoropoff]

mkdocstrings/mkdocstrings#297 uses this to automatically cross-reference base classes. If the base class isn't in the same package, we don't want to warn about an unmapped reference.