Eglot: display completion label when safe

Eglot: display completion label when safe

Originally reported in
https://github.com/joaotavora/eglot/discussions/1141 by "Mintsoup".

Eglot doesn't always show the LSP :label property of a CompletionItem
in the completion candidates.  That is because label is sometimes not
what should be inserted in the buffer in the end, the :insertText
property supercedes it.

But the label is usually more suitable for display nevertheless and if
the LSP CompletionItem contains either a snippet or a textEdit, it's
safe to display the label, since :exit-function will guarantee that a
suitable buffer insertion is performed.

This change reflects that awareness that when a textEdit is available,
it's acceptable to show the label.

* lisp/progmodes/eglot.el (eglot-completion-at-point): Adjust.