Add extra eslint rules

[bonnie57]

Description

1. Using a single import statement per module will make the code clearer because you can see everything being imported from that module on one line.

Example of incorrect code for this rule:

import { merge } from 'module';
import something from 'another-module';
import { find } from 'module'; //error

2. Require default cases in switch statements

Example of incorrect code for this rule:

/*eslint default-case: "error"*/

switch (a) {
    case 1:
        /* code */
        break;
}

3. Require the use of === and !==

Example of incorrect code for this rule:

if (x == 42) { }

4. Require const declarations for variables that are never reassigned after declared

Example of incorrect code for this rule:

// it's initialized and never reassigned.
let a = 3;
console.log(a);