`ZeroCopyBuf` was convenient to use, but sometimes it did hide details
that some copies were necessary in certain cases. Also it made it way to easy
for the caller to pass around and convert into different values. This commit
splits `ZeroCopyBuf` into `JsBuffer` (an array buffer coming from V8) and
`ToJsBuffer` (a Rust buffer that will be converted into a V8 array buffer).
As a result some magical conversions were removed (they were never used)
limiting the API surface and preparing for changes in #19534.
This commit adds support for exporting RSA JWKs in the Web Crypto API.
It also does some minor fixes for RSA JWK imports.
Co-authored-by: Sean Michael Wykes <sean.wykes@nascent.com.br>
