Adds support for serializing bigints.

[MicahZoltu]

I went with serializing bigints as an object with a type and value, but I'm open to other ideas for how to serialize them so they are JSON compatible.

I don't know why I needed to change the type of setInCopy, but the original type definition appears to have been incorrect.

I largely don't understand what most of the properties in parseProps are for, so I just tried to copy other parsers in a way that seemed reasonable. In particular, editable I set to !forceReadonly but this was really just a guess on my part.

I made a few changes to improve the type strictness of JSONValue. If these strictness increases are considered undesirable then everything except the addition of JSONBigint can be removed without causing problems.

Fixes #453

[MicahZoltu]

Note: While I added some tests to verify the functionality works, I did not build/install the plugin to verify that #453 is actually fixed.

[MicahZoltu]

Fixed PR to actually work. Tested with a dapp and I was able to successfully marshal bigint values from my dapp to the extension, edit them, and marshal them back. I tested regular properties, signals, and hooks and they all appear to work properly both directions.

Feedback definitely appreciated. While everything works and tests all pass, I am a small step up from a monkey smashing on a keyboard when it comes to understanding what I'm doing in this codebase. I think I have figured out enough that this PR should work, but someone more familiar should take a look at it to verify it is all reasonable.

Of particular interest, the place that I'm converting from { type: 'bigint', value: '5' } to 5n when writing values back to the app may not be appropriate. It does appear there is one step higher I could go in the call tree that might work for update-prop calls, but having the check in about the same spot for regular, hook, and signals seemed better.

[marvinhagemeister]

Awesome, thanks a bunch for the PR 👍