Rule proposal: no-access-state-in-setstate

[jaaberg]

This rule prevents the use of this.state within this.setState. It is further described in docs/rules/no-access-state-in-setstate.md in the PR, but to summarize: this.setState is async, and referring to this.state within it may lead to race conditions.

This is my first eslint-plugin PR, so I am really happy to get any kind of feedback.

edit:
This is also the recommended way doing it in the react docs:

If the next state depends on the previous state, we recommend using the updater function form, instead

this.setState((prevState) => {
 return {counter: prevState.quantity + 1};
});