Explore tread support

[dbaeumer]

There is now an early specification for threads in WASI and the WASI-SDK as a first implementation of this. We should explore supporting them in the VSCode WASM/WASI host since threads will enable a lot of additional use cases.

[dbaeumer]

Implementation including tests is done.

Still need to publish the extension to the marketplace, including an npm module to use the extension as a libray.

[dbaeumer]

Implementation is done. Closing for now since the thread story will change for preview 2 / 3