Add support for empty expansions (#82)

Fixes https://github.com/tree-sitter/tree-sitter-bash/issues/57

corpus/literals.txt

@@ -57,6 +57,8 @@ echo $# $* $@ $!
 Variable expansions
 =============================
 
+echo ${}
+echo ${#}
 echo ${var1#*#}
 echo ${!abc}
 echo ${abc}
@@ -68,6 +70,8 @@ echo ${abc:
 ---
 
 (program
+  (command (command_name (word)) (expansion))
+  (command (command_name (word)) (expansion))
   (command (command_name (word)) (expansion (variable_name) (word)))
   (command (command_name (word)) (expansion (variable_name)))
   (command (command_name (word)) (expansion (variable_name)))

grammar.js

@@ -489,7 +489,7 @@ module.exports = grammar({
     expansion: $ => seq(
       '${',
       optional(choice('#', '!')),
-      choice(
+      optional(choice(
         seq(
           $.variable_name,
           '=',
@@ -510,7 +510,7 @@ module.exports = grammar({
             ':', ':?', '=', ':-', '%', '-', '#'
           ))
         ),
-      ),
+      )),
       '}'
     ),
 

src/grammar.json

@@ -2137,6 +2137,9 @@
             }
           ]
         },
+        {
+          "type": "CHOICE",
+          "members": [
             {
               "type": "CHOICE",
               "members": [
@@ -2265,6 +2268,11 @@
                 }
               ]
             },
+            {
+              "type": "BLANK"
+            }
+          ]
+        },
         {
           "type": "STRING",
           "value": "}"

src/node-types.json

@@ -604,7 +604,7 @@
     "fields": {},
     "children": {
       "multiple": true,
-      "required": true,
+      "required": false,
       "types": [
         {
           "type": "_primary_expression",