cfg: added cargo/config; add +nightly

[Horki]

@chemicstry
Added +nightly to cargo build

• separate config for flags

[chemicstry]

I was thinking of using .cargo/config.toml like this, but then decided not to, because you can't specify build-std just for one target. All targets are forced to recompile std this way.

It would work if it was just for wasm_thread examples, but build_wasm.sh is more of an example how to build other projects using this library. I don't think there is a workaround for this until wasm atomics are stabilized in rust rust-lang/rust#77839

[Horki]

@chemicstry Ok, seems to make sense;

• still I have updated branch to add +nigthly on build script, so it can ran on other default toolchain