Create a placeholder for WASI threads implementation

[loganek]

This a simpler version of the PR: #1638
To simplify the review, I'll split it into a few smaller changes, starting from this one, which just creates a function stub and a simplistic example.

[yamt]

This a simpler version of the PR: #1638 To simplify the review, I'll split it into a few smaller changes, starting from this one, which just creates a function stub and a simplistic example.

i feel the original version is simple enough.

[loganek]

@yamt it is indeed, but it's also a bit incorrect, and I'm not happy about the quality of that at all - I only made a PR to save my work and give others opportunity to provide higher-level feedback; I actually never intended to merge it as-it-is. As I came back to that work, I thought I'd use it as opportunity to split it into two separate PRs, where the first one is just to provide a template, and the second will be about the actual implementation (with some TODOs, which we address later on - that's why the development will happen on the dev/wasi_threads branch until we're fully ready). Having said that, many thanks for all your comments in the previous PR, I'll take them into account.

[wenyongh]

LGTM