Add `ToUtf8String` and `ToUtf16String` extensions, as well as `From<String>` trait implementations for `PCSTR` and `PCWSTR`.

[ay0ks]

This update adds two extension traits and From<String> implementations for PCSTR/PCWSTR.

use windows::core::{ToUtf8String, ToUtf16String, PCSTR, PCWSTR};

fn main() {
    let source_string = "Hello, World!".to_string();

    let utf8_string = source_string.clone().to_utf8_string();
    let utf16_string = source_string.clone().to_utf16_string();

    // Also
    let utf8_string = PCSTR::from(source_string.clone());
    let utf16_string = PCWSTR::from(source_string.clone());
}

[ay0ks]

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[kennykerr]

Please search the issues or create a new issue before opening a PR to avoid wasted effort.

Related: #3105