allow postcss syntax to be recognized as css

[lazysergey]

Thanks for awesome extension guys, I would like to use it with PostCSS in my projects.

As extension doesn't provide ability to setup mapping in settings json, here's my change to enable brackets highlight in PostCSS. Tested it here with some basic code:

https://user-images.githubusercontent.com/30532154/116292371-4a690500-a79e-11eb-95ec-f28d37520f68.mov

[Wink007]

its really nice idea!!!