PEG.js grammar: Improve the |Code| rule a bit

Instead of matching segments between blocks character by character,
match them as a whole. Also align the style with other similar rules
(e.g. the comment ones).

spec/parser.spec.js

@@ -596,8 +596,9 @@ describe("PEG.js grammar parser", function() {
   /* Canonical Code is " code ". */
   it("parses Code", function() {
     expect('start = "abcd" {a}'        ).toParseAs(actionGrammar("a"));
-    expect('start = "abcd" {{a}}').toParseAs(actionGrammar("{a}"));
-    expect('start = "abcd" {abc}').toParseAs(actionGrammar("abc"));
+    expect('start = "abcd" {abc}'      ).toParseAs(actionGrammar("abc"));
+    expect('start = "abcd" {{a}}'      ).toParseAs(actionGrammar("{a}"));
+    expect('start = "abcd" {{a}{b}{c}}').toParseAs(actionGrammar("{a}{b}{c}"));
 
     expect('start = "abcd" {{}').toFailToParse();
     expect('start = "abcd" {}}').toFailToParse();

src/parser.pegjs

@@ -414,7 +414,7 @@ CodeBlock "code block"
   = "{" code:Code "}" { return code; }
 
 Code
-  = $([^{}] / "{" Code "}")*
+  = $((![{}] SourceCharacter)+ / "{" Code "}")*
 
 /*
  * Unicode Character Categories