Fix internal vnode typings

[pmkroeker]

I think #1979 causes some regression in the internal VNode typings.

The internal VNode type incorrectly extended preact.VNode.

The issue showed up in my projects webpack build. The below changes fix the issues.

[coveralls]

Coverage increased (+0.1%) to 99.768% when pulling 4f54bbb on pmkroeker:fix-internal-vnode-typings into a803e68 on preactjs:master.

[andrewiggins]

Oh interesting. Does your TS build look at internal.d.ts?

[pmkroeker]

I guess it must. I changed the typings in my node_modules and it ran through fine. VSCode also showed an error on the VNode line.

[andrewiggins]

Oh okay, I think I see it now. This fails when using preact/compat and it appears that our TS tests for preact/compat don't run on build like our other tests, which is how this got through.

To be clear, my hesitation is that, internally, VNode.type is null sometimes and VNode.props isn't an always object so those internal types are correct in spirit (obviously still need to figure out how to express that properly in TypeScript though since what I did in #1979 doesn't compile).

Regardless, errors in internal types should never impact consumers, so why that's happening at all is concerning...

[andrewiggins]

Thanks for the report and taking the time to submit a fix! I'll follow up with a PR to run compat TS tests on build so we can catch these before release