feat: create separate package to make reactor modules easier

[mhmd-azeez]

Context

TinyGo doesn't support Reactor modules yet. While you can use WASI functions in Command modules just fine, you'll have to do some gymnastics in order to make them work in Reactor module functions (i.e. exported functions other than main). This PR adds a new package that does the gymnastics for you, you only need to import it.

Notes:

1. Since standard Go compiler doesn't support exporting functions yet, this only works with TinyGo
2. It seems like TinyGo is working on adding official support for Reactor modules, so maybe this is redundant?
3. Tried to use init function for this, but it seems like init is being called too late in the life cycle. The only downside of explicitly exporting _initialize is the fact that wasmtime CLI refuses to run modules that expose both _start and _initialize. But I don't think this is a deal breaker

[mhmd-azeez]

This is related to extism/go-sdk#55

[nilslice]

nicely done!