Add asWinding, isEmpty and addReversePath to SKPath export

[ErikSom]

Adding missing functionality to the WASM export.
I need asWinding to ensure holes are converted from evenodd to winding.
setFillType(fillType) does not alter the winding of holes.

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[skia-codereview-bot]

This PR (HEAD: 345e454) has been imported to Gerrit for code review.

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[kjlubick]

Can you add yourself to the AUTHORS file?

[skia-codereview-bot]

This PR is being closed because review.skia.org/854936 has been merged.