Type the mapDispatch function

[caasi]

Describe the bug
The mapDispatch function is not well typed. When someone want to use the return value of a thunk action, they will get a thunk action in vscode instead of a promise.

Expected behavior
When someone dispatches a thunk action, their IDE should infer the result as a promised value.

Additional context
One should be familiar with advanced TypeScript types to resolve this issue.