Fix PowerShell highlighting of built-in constant variables (#223)

Use a TextMate scope for PowerShell built-in constant variables that most themes actually have defined in tokenColors.

I see that C#, Ruby and Go TextMate files uses variable.other.constant.<language>, so I went with that.

https://github.com/search?q=repo%3Amicrosoft%2Fvscode%20variable.other.constant.&type=code

Author: o-l-a-v

PowerShellSyntax.tmLanguage

@@ -1007,7 +1007,7 @@
 						<key>0</key>
 						<dict>
 							<key>name</key>
-							<string>support.constant.variable.powershell</string>
+							<string>variable.other.constant.powershell</string>
 						</dict>
 						<key>1</key>
 						<dict>
@@ -1291,7 +1291,7 @@
 						<key>0</key>
 						<dict>
 							<key>name</key>
-							<string>support.constant.variable.powershell</string>
+							<string>variable.other.constant.powershell</string>
 						</dict>
 						<key>1</key>
 						<dict>