Expose process windows_process_extensions_main_thread_handle on Windows
~~I did not find any tests in 7d3e03666a/library/std/src/sys/windows/process/tests.rs that actually launch processes, so I haven't added tests for this.~~ I ran the following locally, to check that it works as expected:
```rs
#![feature(windows_process_extensions_main_thread_handle)]
fn main() {
use std::os::windows::process::{ChildExt, CommandExt};
const CREATE_SUSPENDED: u32 = 0x00000004;
let proc = std::process::Command::new("cmd")
.args(["/C", "echo hello"])
.creation_flags(CREATE_SUSPENDED)
.spawn()
.unwrap();
extern "system" {
fn ResumeThread(_: *mut std::ffi::c_void) -> u32;
}
unsafe {
ResumeThread(proc.main_thread_handle());
}
let output = proc.wait_with_output().unwrap();
let str_output = std::str::from_utf8(&output.stdout[..]).unwrap();
println!("{}", str_output);
}
```
Without the feature attribute it wouldn't compile, and commenting the `ResumeThread` line makes it hang forever, showing that it works.
Trakcing issue https://github.com/rust-lang/rust/issues/96723
f689f6582c