fix(bindgen): deconflict instantiation helper locals from WIT import names

[GreenBusDriver]

Addresses #2134
The instantiate() / $init helper hard-coded its locals (promise,
resolve, reject, gen, runNext, ...) as literal text, so a component
importing a WIT function with one of those names produced JS that fails to
load with Identifier 'reject' has already been declared.

Allocate the helper locals through LocalNames like every other generated
binding, so the collision is resolved as reject$1 automatically.

Zulip link: #jco > reject/resolve import name collision w instantiation helper @ 💬