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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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