[web] Avoid using js_util.{jsify,dartify}() in dart2wasm for converting to JS wrappers

[mkustermann]

The js_util.jsify() related code shows up in CPU profile of wonderous.

=> Any SkwasmObjectWrapper object invokes this logic in the constructor and dispose method.

This PR

• makes DomFinalizationRegistryExtension accept JSAny types instead of Dart types and internally converting
  => Callsites can call more precise <>.toJS* extension methods
  => Will avoids extra type checks on the objects when we can call Object.toJSBox directly

• avoids making toJSAnyShallow delegate to toJSAnyDeep in wasm
  => toJSAnyDeep uses js_util.jsify() which is slow
  => We cannot use Object.toJSBox due to it being slower to create JS boxes as it semantically does something different atm (see issue below)
  => Instead use conditional import of dart:_wasm which provides the necessary primitives
  => Similar for going from JS to Dart.

• Avoid calling converting from Dart object to JSAny more than needed (we did the operation twice for each registration and once for unregistration)

Issue dart-lang/sdk#55183

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[mkustermann]

Something fishy going on with the imports - possibly due to this sdk_rewriter.dart tool, I'll have a look later.

[mkustermann]

Not sure why some things fail here and not sure whether it's worthwhile to debug.

Web/Interop team seems open to my suggestion on dart-lang/sdk#55183 / dart-lang/sdk#55187 - if a new API gets added, we can replace this PR by calling the new API.

[mkustermann]

It appears some existing flutter web engine code uses thetoJSAnyShallow/toObjectShallow API despite it actually wanting a deep conversion (it happens to work in dart2js - as dart objects are JS objects as well, but not in dart2wasm)

So I've kept the old implementation and added a toJSWrapper / fromJSWrapper that we use for the finalizer code.

[mkustermann]

@eyebrowsoffire I've made the PR now only affect finalizer code (which I saw on cpu profile), ( thereby not affecting other .toJSAnyShallow call sites that may actually require deep). Let me know if it looks ok to you

[eyebrowsoffire]

LGTM!

[mkustermann]

Thanks, @eyebrowsoffire !