Handle shorthand file redirect command substitutions

Fixes #34

corpus/literals.txt

@@ -159,6 +159,7 @@ Command substitutions
 echo `echo hi`
 echo `echo hi; echo there`
 echo $(echo $(echo hi))
+echo $(< some-file)
 
 ---
 
@@ -175,7 +176,10 @@ echo $(echo $(echo hi))
       (command_name (word))
       (command_substitution (command
         (command_name (word))
-        (word)))))))
+        (word))))))
+  (command
+    (command_name (word))
+    (command_substitution (file_redirect (word)))))
 
 =============================
 Process substitutions

grammar.js

@@ -472,6 +472,7 @@ module.exports = grammar({
 
     command_substitution: $ => choice(
       seq('$(', $._statements, ')'),
+      seq('$(', $.file_redirect, ')'),
       prec(1, seq('`', $._statements, '`'))
     ),
 

src/grammar.json

@@ -2000,6 +2000,23 @@
             }
           ]
         },
+        {
+          "type": "SEQ",
+          "members": [
+            {
+              "type": "STRING",
+              "value": "$("
+            },
+            {
+              "type": "SYMBOL",
+              "name": "file_redirect"
+            },
+            {
+              "type": "STRING",
+              "value": ")"
+            }
+          ]
+        },
         {
           "type": "PREC",
           "value": 1,