C#: Fix wrong highlighting when three slashes appear inside string. Fix

#1091

components/prism-csharp.js

@@ -1,8 +1,14 @@
 Prism.languages.csharp = Prism.languages.extend('clike', {
 	'keyword': /\b(abstract|as|async|await|base|bool|break|byte|case|catch|char|checked|class|const|continue|decimal|default|delegate|do|double|else|enum|event|explicit|extern|false|finally|fixed|float|for|foreach|goto|if|implicit|in|int|interface|internal|is|lock|long|namespace|new|null|object|operator|out|override|params|private|protected|public|readonly|ref|return|sbyte|sealed|short|sizeof|stackalloc|static|string|struct|switch|this|throw|true|try|typeof|uint|ulong|unchecked|unsafe|ushort|using|virtual|void|volatile|while|add|alias|ascending|async|await|descending|dynamic|from|get|global|group|into|join|let|orderby|partial|remove|select|set|value|var|where|yield)\b/,
 	'string': [
-		/@("|')(\1\1|\\\1|\\?(?!\1)[\s\S])*\1/,
-		/("|')(\\?.)*?\1/
+		{
+			pattern: /@("|')(\1\1|\\\1|\\?(?!\1)[\s\S])*\1/,
+			greedy: true
+		},
+		{
+			pattern: /("|')(\\?.)*?\1/,
+			greedy: true
+		}
 	],
 	'number': /\b-?(0x[\da-f]+|\d*\.?\d+f?)\b/i
 });

tests/languages/csharp/issue1091.test

@@ -0,0 +1,12 @@
+@"file:///"
+
+----------------------------------------------------
+
+[
+	["string", "@\"file:///\""]
+]
+
+----------------------------------------------------
+
+Checks that three slashes inside a string do not break highlighting.
+See #1091.