Support goto-{declaration, implementation, typedefinition}

Support goto-{declaration, implementation, typedefinition}

Closes https://github.com/joaotavora/eglot/issues/302.

* eglot.el (eglot--xref-definitions-method): New variable.
(xref-backend-definitions): Use it.
(eglot-find-declaration, eglot-find-implementation,
eglot-find-typeDefinition): New functions.

* README.md (Language features): Add new capabilities.

* eglot.el (eglot-client-capabilities): Add new capabilities.
(eglot-ignored-server-capabilites): Add new capability.