applies issue PowerShell#167 part 1

permits whitespace to follow here-string start token.

Author: msftrncs

PowerShellSyntax.tmLanguage

@@ -134,7 +134,7 @@
 		</dict>
 		<dict>
 			<key>begin</key>
-			<string>\@"(?=$)</string>
+			<string>\@"\s*$</string>
 			<key>end</key>
 			<string>^"@</string>
 			<key>name</key>
@@ -157,7 +157,7 @@
 		</dict>
 		<dict>
 			<key>begin</key>
-			<string>\@'(?=$)</string>
+			<string>\@'\s*$</string>
 			<key>end</key>
 			<string>^'@</string>
 			<key>name</key>