WebR - Expreimental

[WebReflection]

This MR brings in most basic support for WebR with the following known caveats:

• there is no synchronous version of WebR, everything is inevitably async
• they decided to run out of the box in a worker as opposite of focusing on WASM <-> JS bindings and let actually consumers decide when the worker is desired (like every other runtime in here that works seamlessly and it can be moved into a worker still with full 1:1 bindings support, WebR breaks if it starts within a Worker for what I could test)
• because everything is inevitably async, there is no difference between run and runAsync
• because they offer various conversion of their returned data, it's a consumer exercise to explicitly (await result.toJs()).values or any other variant of the result
• the FS currently is also async even if the Emscripten FS is sync ... on top of that, cwd() is missing so the writeFile within the interpreter wrap doesn't work and it throws an error
• the JS module cannot be solved until generic JS objects are properly translated in WebR, which is not currently the case ... neither object literals or functions can apparently land so no import polyscript (or any other) is allowed
• for the same reason the js_modules is not working neither
• the worker attribute is useless, redundant, it actually breaks ... WebR always runs in a worker so do not use it
• because everything is async, events as listeners cannot work as expected or do anything meaningful, also because events cannot cross JS -> WebR boundaries as their data.frame is not supported

That's it, that's the current state of WebR:

• we can evaluate and define R code as content or external file
• we can invoke those public functions from JS via runAsync (or run but it's async anyway)
• we need to transform the returned value also async
• we can pass numbers or primitives, array of primitives, or explicit data.frame values (tables) to R when invoking

/cc @ntoll

[ntoll]

Bravissimo!