Beginnings of the SkSL DSL framework

The code at this point doesn't do anything useful, but establishes some
of the basic types and patterns.

Change-Id: I580a9e75ffa3162879893450fb7d1f0905a10687
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/350697
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Auto-Submit: Ethan Nicholas <ethannicholas@google.com>
14 files changed