`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.
A few easy migrations of module code from the runtime to the module map.
The module map already has a few places where it needs a handle scope,
so we're not coupling it any further with the v8 runtime.
`init_runtime_module_map` is replaced with an option to reduce API
surface of JsRuntime.
`module_resolve_callback` now lives in the `ModuleMap` and we use a
annex data to avoid having to go through the `Rc<RefCell<...>>` stored
in the `JsRuntime`'s isolate.
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.
