Multiple servers per project are possible

Multiple servers per project are possible

A server manages a specific major-mode within a project.

* eglot.el (eglot--processes-by-project): Add docstring.
(eglot--current-process): Search new eglot--processes-by-project
format.
(eglot--major-mode): New variable.
(eglot--moribund, eglot--project): Update docstring.
(eglot--project-short-name, eglot--all-major-modes): New helpers.
(eglot--connect): Rework.
(eglot-new-process): Rework severely.
(eglot--command-history): New variable.
(eglot--process-sentinel): Use new
eglot--processes-by-project. Update mode line.
(eglot-editing-mode): Don't start processes, just suggest it.