* eglot.el (eglot--current-buffer-TextDocumentPositionParams): New
helper.
(xref-backend-identifier-completion-table): Refactor a bit.
(xref-backend-identifier-at-point): Use when-let and
eglot--current-buffer-TextDocumentPositionParams
(xref-backend-definitions, xref-backend-references): Refactor a
bit.
(eglot-completion-at-point): Use
eglot--current-buffer-TextDocumentPositionParams
(eglot-eldoc-function): Rewrite to handle
textDocument/documentHighlight.
(eglot--highlights): New variable.
(eglot--client-capabilities): Update with support for documentHighlight.