Adds support for comments in JSON (#1595)

While JSON doesn't explicitly support comments, they're commonly used
and there are supersets of JSON that do make comments available. PrismJS
is not a linter, and this is a useful feature to include when displaying JSON.

components/prism-json.js

@@ -1,5 +1,9 @@
 Prism.languages.json = {
-	'property': /"(?:\\.|[^\\"\r\n])*"(?=\s*:)/i,
+	'comment': /\/\/.*|\/\*[\s\S]*?(?:\*\/|$)/,
+	'property': {
+		pattern: /"(?:\\.|[^\\"\r\n])*"(?=\s*:)/i,
+		greedy: true
+	},
 	'string': {
 		pattern: /"(?:\\.|[^\\"\r\n])*"(?!\s*:)/,
 		greedy: true

tests/languages/json/comment_feature.test

@@ -0,0 +1,27 @@
+{
+	// Line comment
+	"//": "//",
+
+	/* Block comment */
+	"/*": "*/"
+}
+
+----------------------------------------------------
+
+[
+	["punctuation", "{"],
+	["comment", "// Line comment"],
+	["property", "\"//\""],
+	["operator", ":"],
+	["string", "\"//\""],
+	["punctuation", ","],
+	["comment", "/* Block comment */"],
+	["property", "\"/*\""],
+	["operator", ":"],
+	["string", "\"*/\""],
+	["punctuation", "}"]
+]
+
+----------------------------------------------------
+
+Checks for single-line and multi-line comments.