Eglot: use new xref-find-extra feature of Xref

Eglot: use new xref-find-extra feature of Xref

This removes the need of some Eglot hacks to implement the commands
eglot-find-declaration, eglot-find-implementation, and
eglot-find-typeDefinition.

It also allows the new command xref-find-extra to find different kinds
of definitions.

* lisp/progmodes/eglot.el (eglot--xref-kinds-alist): New variable.
(eglot-find-declaration, eglot-find-implementation)
(eglot-find-typeDefinition): Use xref-find-extra.
(xref-backend-definitions): Minor tweak.
(xref-backend-references): Don't use eglot--lsp-xref-refs.
(eglot--lsp-xref-helper): Delete this helper.
(eglot--lsp-xref-refs): Delete this variable.
(xref-backend-extra-kinds, xref-backend-extra-defs): New implementation.