webassembly/objjsproxy: Lookup attributes without testing they exist.

In JavaScript when accessing an attribute such as `obj.attr` a value of
`undefined` is returned if the attribute does not exist.  This is unlike
Python semantics where an `AttributeError` is raised.  Furthermore, in some
cases in JavaScript (eg a Proxy instance) `attr in obj` can return false
yet `obj.attr` is still valid and returns something other than `undefined`.
So the source of truth for whether a JavaScript attribute exists is to just
right away attempt `obj.attr`.

To more closely match these JavaScript semantics when proxying a JavaScript
object through to Python, change the attribute lookup logic on a `JsProxy`
so that it immediately attempts `obj.attr` instead of first testing if the
attribute exists via `attr in obj`.

This allows JavaScript objects which dynamically create attributes to work
correctly on the Python side, with both `obj.attr` and `obj["attr"]`.  Note
that `obj["attr"]` already works in all cases because it immediately does
the subscript access without first testing if the attribute exists.

As a benefit, this new behaviour matches the Pyodide behaviour.

Signed-off-by: Damien George <damien@micropython.org>

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