New eglot-ensure to put in a major-mode's hook

New  eglot-ensure to put in a major-mode's hook

* eglot.el (Commentary): Mention eglo-ensure.
(eglot--connect): Rearrange args.
(eglot--guess-contact): Rename from eglot--interactive.
(eglot): Use eglot--guess-contact.
(eglot, eglot-reconnect): Rearrange call to eglot--connect.
(eglot-ensure): New command to put in mode hook.

* eglot-tests.el (eglot--tests-connect): New helper.
(auto-detect-running-server, auto-reconnect, rls-watches-files)
(rls-basic-diagnostics, rls-hover-after-edit, rls-rename)
(basic-completions, hover-after-completions): Use it.

GitHub-reference: close https://github.com/joaotavora/eglot/issues/17