docs(core): add JSDocs for exported module members

[jviide]

This pull request adds rudimentary JSDocs for all exported members of the core module.

The JSDoc contents are mostly mixed & matched from the README.md and the Preact website.

I renamed the compute arguments to fn and the internal _compute properties to _fn to align with external documentation. The mangle.json needed to be updated accordingly, with some unrelated formatting changes by the automation.

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Latest commit: 582f074

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[marvinhagemeister]

Sweet 👍