Add missing variadic bindings for string methods

[RReverser]

• JsString::from_code_point - allows to create JS strings using slice of codes in WASM memory.
• JsString::from_char_code - same as above, but also uses updated signature with u16 instead of u32 (partially helps with JsString::from_char_code1 and family should take u16 not u32 #1460 at least for the new binding).

[RReverser]

127 | #[wasm_bindgen]
    | ^^^^^^^^^^^^^^^ the trait `wasm_bindgen::convert::traits::IntoWasmAbi` is not implemented for `&[wasm_bindgen::JsValue]`

Ah hmm... I should have checked these.

Is there any particular reason these slices are not allowed? (while Box<[JsValue]> is)

[RReverser]

I've added support for [JsValue] slices since it seemed easy to do so (resulting in removing more code than added), but let me know if that's not desired for some reason.

[RReverser]

Oh, hmm, currently bindings assume that they can drop every item of the slice. I see, it's not as easy as seemed initially.

[RReverser]

Removed the problematic bindings for now, and kept just String::fromCharCode and String::fromCodePoint instead.

[alexcrichton]

👍