Implement tcp autostart/autoconnect (and support ruby's solargraph)

Implement tcp autostart/autoconnect (and support ruby's solargraph)

* README.md (Installation and usage): Mention support for
Solargraph
(Connecting via TCP): New section
(Connecting automatically): New section

* eglot.el (eglot-server-programs): Add ruby-mode.
Overhaul docstring.
(eglot-lsp-server): Add inferior-process slot.
(eglot--on-shutdown): Kill any autostarted inferior-process
(eglot--guess-contact): Allow prompting with :autoport parameter.
(eglot--connect): Consider :autoport case.
(eglot--inferior-bootstrap): New helper.