MutationState type should be more specific, to allow narrowing to match useQuery

[gavinwahl]

Conditionals on mutation status should narrow the type of data. For example,

mutation.status === 'success' && mutation.data

mutation.data can not be undefined after the guard, but the type doesn't reflect this. The object returned by useQuery does support narrowing like this, so useMutation should too.

[TkDodo]

Sounds good. Would you like to open a PR?

[pobch]

Also, the error status should support type narrowing too:

mutation.status === `error` && mutation.error