Consider adding JsValue::as_char

[RReverser]

Motivation

Currently it's not too easy to convert a single char, which is represented in JS as a string, and involves either getting a heap-allocated String and then extracting a char, or doing manual conversions that use char_code_at / code_point_at which currently don't have convenient types (see #1362), and also checking the length to make sure it's indeed a char and not an arbitrary string.

Proposed Solution

Add as_char on JsValue that would make such conversions easier by performing length check and .codePointAt on JS side, passing Option<u32> back to Rust and converting back to char using char::from_u32 internally.

Alternatives

Wait for fixes to #1362, and then ask users to use js-sys and perform conversion manually, but the length check is still non-trivial to implement correctly (because JS length can be 1 or 2 for a single-codepoint string) and feels like belongs to the core of wasm-bindgen.

[RReverser]

For the reference, this is what my current conversion code looks like:

let maybe_char = match s.length() {
    1 => {
        // Should be valid `u16` because we checked the length.
        std::char::from_u32(s.char_code_at(0) as u32)
    }
    2 => {
        // Should be a valid `u32` by now.
        let cp = s.code_point_at(0).as_f64().unwrap() as u32;
        // This is a char only if it consists of two surrogates.
        if cp > 0xFFFF {
            std::char::from_u32(cp)
        } else {
            None
        }
    }
    _ => None,
};

While there can be several other variations (including always using code_point_at), it's still too verbose and tricky to get right to leave this to the user IMO.