Add support for locationlink

Add support for locationlink

Fix https://github.com/joaotavora/eglot/issues/711.

LocationLink was added in version 3.14 of the protocol and is
sometimes used in lieu of Location for definition- and
reference-related requests.

* eglot.el (eglot--lsp-interface-alist): Update with LocationLink.
(eglot-client-capabilities): Advertise
textDocument.{definition,declaration,implementation,typeDefinition}.linkSupport.
(eglot--lsp-xrefs-for-method): Accept LocationLinks.

Co-authored-by: João Távora <joaotavora@gmail.com
GitHub-reference: close https://github.com/joaotavora/eglot/issues/712