Add support for parsing @apply rules.

[rictic]

Follows the proposed (WIP) spec as it exists today at https://tabatkins.github.io/specs/css-apply-rule/#using

Fixes #8

[rictic]

I've signed the Microsoft CLA, and contributed to other Microsoft repos, e.g. microsoft/TypeScript-DOM-lib-generator#151

[rictic]

Please let me know anything I can do to help this land!

I tried to make the change minimal, and follow the conventions of the repo.

[aeschli]

@rictic Changes look good. Any reason why endOfAtApply and semicolonPosition are remembered?

[rictic]

Just trying to follow the conventions. AbstractDeclaration tracks colonPosition and semicolonPosition and those were the closest analogs for @apply. Might have been cargo culting though, I didn't track down how those were used.

[rictic]

If you like I could drop endOfAtApply and semicolonPosition, or hook them into other parts of the code. Just let me know!

[abdonrd]

Yay! Thanks @rictic!

[aeschli]

Thanks @rictic . I polished the code a bit and simplified it. Removed the endOfAtApply and semicolonPosition, and also took away the test for --. This can better be done by a linter extension.

Thanks a lot for the contribution and sorry for the wait.