Feature: Implement Goto Definition (textDocument/definition) handler

[bryphe]

This is tracking implementation of the textDocument/definition handler.

For Markdown files, this is especially useful when there is a relative URL called out, like:

[example](./examples/example.md). 

Ideally, when the cursor is over example, we could send a response that joins the path of the current file with the relative file path as the location.

In addition, it would be great to handle cases where an anchor/marker is specified, like:

[first example](./examples/example.md#first)

In this case, as part of the definition response, we should send a Range that corresponds to the first anchor in example.md

[bryphe]

Actually - I wonder if this is more appropriate to use the textDocument/documentLink request?

[DeltaEvo]

A textDocument/documentLink seems more appropriate because DocumentLink links to an internal or external resource so we can cover all type of links in a markdown document.

But I don't know how this is implemented in the Client's Side and if the link refer to an internal ressource, the Client handle it like textDocument/definition by providing a goto ?

If not we can implement textDocument/definition too

PS: In vscode the link is underlined and a Ctrl+Click open it

[DeltaEvo]

For information vscode-html-languageservice only implement textDocument/documentLink by searching all href and src in the attributes of the nodes in the document