Add support for R languageserver (#3370)

doc/ale-r.txt

@@ -2,6 +2,29 @@
 ALE R Integration                                               *ale-r-options*
 
 
+===============================================================================
+languageserver                                           *ale-r-languageserver*
+
+g:ale_r_languageserver_cmd                         *g:ale_r_languageserver_cmd*
+                                                   *b:ale_r_languageserver_cmd*
+  Type: |String|
+  Default: `'languageserver::run()'`
+
+  This option can be configured to change the execution command for
+  languageserver.
+
+  See the languageserver documentation for more options.
+
+
+g:ale_r_languageserver_options                 *g:ale_r_languageserver_options*
+                                               *b:ale_r_languageserver_options*
+  Type: |Dictionary|
+  Default: `{}`
+
+  This option can be configured to change settings for languageserver. See the
+  languageserver documentation for more information.
+
+
 ===============================================================================
 lintr                                                             *ale-r-lintr*
 

doc/ale-supported-languages-and-tools.txt

@@ -389,6 +389,7 @@ Notes:
   * `qmlfmt`
   * `qmllint`
 * R
+  * `languageserver`
   * `lintr`
   * `styler`
 * Racket

doc/ale.txt

@@ -2854,6 +2854,7 @@ documented in additional help files.
   qml.....................................|ale-qml-options|
     qmlfmt................................|ale-qml-qmlfmt|
   r.......................................|ale-r-options|
+    languageserver........................|ale-r-languageserver|
     lintr.................................|ale-r-lintr|
     styler................................|ale-r-styler|
   reasonml................................|ale-reasonml-options|