add optional leading comment to shell_command

Previously, if a shell command started with a comment line, parsing
would fail because shell_command expected the first line to be a shell
line. This adds an optional comment to the beginning of the
shell_command to handle this correctly.

corpus/combo.txt

@@ -38,3 +38,40 @@ RUN echo hello \
 			(comment)
 			(comment)
 			(shell_fragment))))
+
+==================
+Run with immediate line continuation
+==================
+
+RUN \
+	# comment
+	world
+
+---
+
+(source_file
+	(run_instruction
+        (line_continuation)
+		(shell_command
+			(comment)
+			(shell_fragment))))
+
+==================
+Run with immediate continuation and comment
+==================
+
+RUN \
+  echo foo \
+  # comment
+  echo foo
+
+---
+
+(source_file
+	(run_instruction
+        (line_continuation)
+		(shell_command
+            (shell_fragment)
+            (line_continuation)
+			(comment)
+			(shell_fragment))))

grammar.js

@@ -180,7 +180,9 @@ module.exports = grammar({
       seq(
         field("name", $._env_key),
         token.immediate("="),
-        optional(field("value", choice($.double_quoted_string, $.unquoted_string)))
+        optional(
+          field("value", choice($.double_quoted_string, $.unquoted_string))
+        )
       ),
 
     _spaced_env_pair: ($) =>
@@ -211,7 +213,8 @@ module.exports = grammar({
         )
       ),
 
-    image_name: ($) => seq(
+    image_name: ($) =>
+      seq(
         choice(/[^@:\s\$-]/, $.expansion),
         repeat(choice(/[^@:\s\$]+/, $.expansion))
       ),
@@ -249,11 +252,13 @@ module.exports = grammar({
 
     shell_command: ($) =>
       seq(
+        repeat($._comment_line),
         $.shell_fragment,
         repeat(
           seq(
             alias($.required_line_continuation, $.line_continuation),
-            repeat($._comment_line), $.shell_fragment
+            repeat($._comment_line),
+            $.shell_fragment
           )
         )
       ),

src/grammar.json

@@ -1251,6 +1251,13 @@
     "shell_command": {
       "type": "SEQ",
       "members": [
+        {
+          "type": "REPEAT",
+          "content": {
+            "type": "SYMBOL",
+            "name": "_comment_line"
+          }
+        },
         {
           "type": "SYMBOL",
           "name": "shell_fragment"