fix: initialise default load options in Magika class (npm/js)

[TheMythologist]

This fixes the following error when using magika.load():

TypeError: Cannot destructure property 'modelURL' of 'undefined' as it is undefined.

The current workaround is to use magika.load({}), which does not seem intuitive, and is also not what's described in the documentation.

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As a side note, I was thinking of creating a constructor method in the Magika class that will perform the Magika.load operation by default, passing in the load options into the constructor. What do you think?

[reyammer]

Thanks for the PR! @invernizzi: thoughts?

[invernizzi]

Absolutely, thanks!

[reyammer]

Reopening as there was a question on creating a constructor: "As a side note, I was thinking of creating a constructor method in the Magika class that will perform the Magika.load operation by default, passing in the load options into the constructor. What do you think?"

This feels similar to what we do in the python package, which seems reasonable.

@TheMythologist: do you confirm it's basically the same?

@invernizzi: Thoughts?

If we need to change the API, better to do it now / early on...