feat: query-style, generic useFlag hook

[toddbaert]

This PR adds a new useFlag hook. This hook:

• returns an object supporting a react "query-style" API with relevant properties: const { value: newMessage, isError, reason, errorCode, isAuthoritative, type } = useFlag('new-message', true);
• supports ergonomic generics:

const { value: boolVal }: FlagQuery<boolean> = useFlag('bool-flag', true);
const { value: stringVal }: FlagQuery<string> = useFlag('string-flag', 'string');
const { value: objVal }: FlagQuery<{greeting: string}> = useFlag('obj-flag', { greeting: 'hi' });

• supports all the same options and features as other hooks (suspense, etc)

This PR fixes some type bugs as well, and adds to the readme.

DEPENDS ON: #898

[juanparadox]

Just some comments 😄

[moredip]

woop!