Fixes #19568
Values are not coerced to the desired type during deserialisation. This
makes serde_v8 stricter.
---------
Co-authored-by: Bartek Iwańczuk <biwanczuk@gmail.com>
`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 is a quick first refactoring to split the tests out of runtime and
move runtime-related code to a top-level runtime module.
There will be a followup to refactor imports a bit, but this is the
major change that will most likely conflict with other work and I want
to merge it early.