Design Doc: TC39 Source Phase Import

[legendecas]

The goal is to implement the TC39 stage 3 proposal Source Phase Import, which enables importing WebAssembly.Module (and others to be defined) with ECMAScript module syntax.

Design Doc: https://docs.google.com/document/d/1yetUlxl_yHk0ooT-hg9ylJeQ6Q77_Ttm777tHbclwqg/edit?usp=sharing

Please take a look at your leisure.