Make children optional in Provider's typings

[shicks]

Without this, it's an error to write <Context.Provider value={...}> since TypeScript expects a children prop to be explicitly provided. It's easy enough to work around by adding children={[]}, but this shouldn't be necessary.

[coveralls]

coverage: 99.463%. remained the same
when pulling 265e97d on shicks:patch-1
into 08ff289 on preactjs:main.