[PathKit] Write more complete docs and clean up API to be consistent

Breaking Changes (should be minor, as it's mostly just things
for testing):
 - PathKit.ApplyPathOp should have returned a new SkPath, but didn't.
It now does and is named "MakeFromOp", which makes the convention of
"Have 'make' in name, needs delete" more consistent.
 - PathKit.FromCmds(arr) now only needs to take the JS Array and
will handle the TypedArrays under the hood. If clients want to deal
with TypedArrays themselves, they can use _FromCmds(ptr, len) directly.
 - PathKit.MakeLTRBRect is now just PathKit.LTRBRect. The thing
returned is a normal JS Object and doesn't need delete().

As per custom with v0 apps, we are updating the minor version
to v0.3.0 to account for breaking changes.


Docs-Preview: https://skia.org/?cl=147960
Bug: skia:8216
Change-Id: Ia3626e69f3e97698fc62a6aee876af005e29ffca
Reviewed-on: https://skia-review.googlesource.com/147960
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Heather Miller <hcm@google.com>
13 files changed