wasm: isolate nodejs-specific logic with the isNode flag in dyld

As we move towards supporting running the dyld script in the browser,
this commit implements the isNode module-level binding which is true
if dyld is running in nodejs. The nodejs-specific bits are gated under
isNode.

For the browser case, this commit introduces @bjorn3/browser_wasi_shim
as the wasi implementation; we already use it in quite a few projects
and it simply works.