design: add go:wasmexport proposal
design: add go:wasmexport proposal

Detailed proposal about issue https://golang.org/issue/42372

For golang/go#42372

Change-Id: I48f5d1d6b055349743e7018f8bc3ebe2fa2bcda1
GitHub-Last-Rev: d10c2d44921519cd0b1c2bffeaf184815b4720f8
GitHub-Pull-Request: golang/proposal#31
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/278692
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/proposal,golang/proposal