skip ReadonlyMap and ReadonlySet types when not available

[phryneas]

This should fix #624 .

Instead of my initial idea of providing interfaces with the same shape as ReadonlyMap, ReadonlySet, WeakMap and WeakSet (which would also have worked, but probably required a lot more duplicate code), this is now following a different approach:

It checks if the types are any (which they would fall back to if they were not present) or a keyless interface (which at least ReadonlyMap and ReadonlySet would be if the node types are installed), in which case they fall back to void, which does not trigger the conditional.

The check for any is done by true | false extends (T extends never ? true : false) - every other type will either be true or false, but since any takes both paths, it becomes true | false.

The check for the empty interface is done by keyof T extends never - everything except the empty interface has some keys, including primitives like string and number (which will have the keys of their prototype).

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[mweststrate]

Thanks for the PR! Looks fine at first sight and clever trick :). However, if it turns out type checking becomes more expensive or there is other fallout, the changes might get referred.

[aleclarson]

🎉 This PR is included in version 7.0.11 🎉

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