DEV: Add a way to observe stores

[thetarnav]

• Adds a dev-only ability to observe stores by adding a new $ON_UPDATE symbol.
• the dev APIs are bundled into a DEV object, as in the core module (not sure if it should be called STORE_DEV to avoid conflicts, but it's probably whatever)
• Some typings got cleaned up too (nothing in public APIs though)
• Added a jsdoc comment to unwrap because why not

Draft because I still want to confirm if this is enough, and maybe add some unit tests.

[coveralls]

Pull Request Test Coverage Report for Build 3388525518

• 11 of 11 (100.0%) changed or added relevant lines in 3 files are covered.
• 14 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.06%) to 89.15%

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Files with Coverage Reduction	New Missed Lines	%
packages/solid/store/src/store.ts	1	95.99%
packages/solid/src/render/component.ts	13	71.52%

Totals
Change from base Build 3287627654:	0.06%
Covered Lines:	1284
Relevant Lines:	1370

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💛 - Coveralls

[thetarnav]

Alright, I think this one's ready
I've changed the API to use a global listener, similar to the existing afterUpdate and afterRootCreate hooks.
This will allow for more flexibility and storing updates without pre-patching anything.