Handle backquoted identifiers [grammar and tests]

Author: keynmol

grammar.js

@@ -68,7 +68,7 @@ module.exports = grammar({
     [$._indentable_expression, $.do_while_expression],
   ],
 
-  word: $ => $.identifier,
+  word: $ => $._plainid,
 
   rules: {
     compilation_unit: $ => repeat($._top_level_definition),
@@ -778,7 +778,12 @@ module.exports = grammar({
     ),
 
     // TODO: Include operators.
-    identifier: $ => /[a-zA-Z_]\w*/,
+    _plainid: $ => /[a-zA-Z_]\w*/,
+    _backquoted_id: $=> /`[^\n`]+`/,
+    identifier: $ => choice(
+      $._plainid, 
+      $._backquoted_id
+    ),
 
     wildcard: $ => '_',