Add make target for building wasmer-api with js feature as cdylib

[maminrayej]

Since Rust 1.59.0, using:

#![cfg_attr(feature = "js", crate_type = "cdylib")]

has been deprecated. Seems like in Rust 1.84.0, this is going to be a hard error, so building the wasmer crate with the js feature enabled, will result in an error (and does result an error now if you try to build with the latest nightly).

I've tried multiple ways to solve this issue, namely:

• Setting a conditional cfg in lib/api/Cargo.toml to set the crate-type to cdylib when the js feature is enabled. This did not work. Cargo will emit a warning that such a config is just ignored. You can also confirm that by seeing that wasmer.wasm was not generated.
• I tried to pass flags to rustc via lib/api/build.rs and RUSTFLAGS, but neither worked.

Finally I've settled on the solution proposed by this PR. For convenience, I added a make target for this specific kind of build. I can confirm that running this target with the latest nightly, correctly produces the desired wasmer.wasm file.

Resolves #5214.