applies issue PowerShell#167 part 3a, simplify regex

Author: msftrncs

PowerShellSyntax.tmLanguage

@@ -1509,7 +1509,7 @@
 		<key>doubleQuotedString</key>
 		<dict>
 			<key>begin</key>
-			<string>(?&lt;!(?&lt;!`)")"</string>
+			<string>"</string>
 			<key>beginCaptures</key>
 			<dict>
 				<key>0</key>