comments must start at beginning of line

Previously, this grammar allowed comments to start partway through
lines, but per the Dockerfile spec, that is not valid.

corpus/comment.txt

@@ -8,18 +8,3 @@ Full line comment
 
 (source_file
 	(comment))
-
-==================
-Comment after instruction
-==================
-
-ADD /src ./dst # Test comment
-
----
-
-(source_file
-	(add_instruction
-		(path)
-		(path))
-	(comment))
-

grammar.js

@@ -1,10 +1,10 @@
 module.exports = grammar({
   name: "dockerfile",
 
-  extras: ($) => [$.comment, /\s+/, "\\\n"],
+  extras: ($) => [/\s+/, "\\\n"],
 
   rules: {
-    source_file: ($) => repeat(seq($._instruction, "\n")),
+    source_file: ($) => repeat(seq(choice($._instruction, $.comment), "\n")),
 
     _instruction: ($) =>
       choice(

src/grammar.json

@@ -7,8 +7,17 @@
         "type": "SEQ",
         "members": [
           {
-            "type": "SYMBOL",
+            "type": "CHOICE",
-            "name": "_instruction"
+            "members": [
+              {
+                "type": "SYMBOL",
+                "name": "_instruction"
+              },
+              {
+                "type": "SYMBOL",
+                "name": "comment"
+              }
+            ]
           },
           {
             "type": "STRING",
@@ -1488,10 +1497,6 @@
     }
   },
   "extras": [
-    {
-      "type": "SYMBOL",
-      "name": "comment"
-    },
     {
       "type": "PATTERN",
       "value": "\\s+"

src/node-types.json

@@ -602,6 +602,10 @@
           "type": "cmd_instruction",
           "named": true
         },
+        {
+          "type": "comment",
+          "named": true
+        },
         {
           "type": "copy_instruction",
           "named": true