Fix build-wasm on Windows

[aminya]

Fixes #434

Tested on tree-sitter-python and it generates the correct wasm file.

[maxbrunsfeld]

The emcc.bat thing looks good. I left one question.

[maxbrunsfeld]

Could you explain the reason for using the which crate here, compared just checking the result of Command::new, since it already looks up the command on your PATH?

[aminya]

Built-in Rust's Command apparently has a buggy behavior.

Running tree-sitter build-wasm with the following code

fn get_emcc_path() -> Result<String> {
    let emcc_bin = if cfg!(windows) {
                        "emcc.bat"
                    } else {
                        "emcc"
                    };
    if Command::new(emcc_bin).output().is_ok() {
        return Ok(emcc_bin.to_string());
    }
    return Error::err("emcc binary not found".to_string());
}

Results in this error:

emcc command failed - C:\Users\aminy\AppData\Local\emsdk\python\3.9.2-1_64bit\python.exe: can't open file 'C:\Users\aminy\tree-sitter-python\emcc.py': [Errno 2] No such file or directory

It seems that emcc.bat is spawned from the current working directory, and so emcc.py is not found throwing an error.

[maxbrunsfeld]

Interesting. Thanks for explaining.

[aminya]

You're welcome