Close joaotavora#379: add built-in support for TeX and friends

plain-tex-mode and latex-mode are derived from tex-mode.  Some other
TeX-related modes are not, so they require an explicit mention in
eglot-server-programs.

* README.md (Connecting to a server): Add Digestif to the list

* eglot.el (eglot-server-programs): Add Digestif for TeX-related modes

Copyright-paperwork-exempt: yes

README.md

@@ -43,6 +43,7 @@ for the language you're using. Otherwise, it prompts you to enter one.
 * Elixir's [elixir-ls][elixir-ls]
 * Ada's [ada_language_server][ada_language_server]
 * Scala's [metals][metals]
+* TeX/LaTeX's [Digestif][digestif]
 
 I'll add to this list as I test more servers. In the meantime you can
 customize `eglot-server-programs`:
@@ -457,3 +458,4 @@ Under the hood:
 [news]: https://github.com/joaotavora/eglot/blob/master/NEWS.md
 [ada_language_server]: https://github.com/AdaCore/ada_language_server
 [metals]: http://scalameta.org/metals/
+[digestif]: https://github.com/astoff/digestif

eglot.el

@@ -105,7 +105,9 @@ language-server/bin/php-language-server.php"))
                                 (dart-mode . ("dart_language_server"))
                                 (elixir-mode . ("language_server.sh"))
                                 (ada-mode . ("ada_language_server"))
-                                (scala-mode . ("metals-emacs")))
+                                (scala-mode . ("metals-emacs"))
+                                ((tex-mode context-mode texinfo-mode bibtex-mode)
+                                 . ("digestif")))
   "How the command `eglot' guesses the server to start.
 An association list of (MAJOR-MODE . CONTACT) pairs.  MAJOR-MODE
 is a mode symbol, or a list of mode symbols.  The associated