Update shBrushPowerShell.js

Fixed a bug with empty double quoted strings ("") which would bleed ignoring the closing quote.
Fixed a bug where double quoted strings ending in "`" would incorrectly show as closed strings.  Now it works as intended.
before empty strings ("") would bleed and it would also incorrectly mark "`" as being a closed string when it needs to be "`"" to be a closed string.