add prettier for auto formatting (#231)

* [Prettier] Add "prettier"

Makes the formatting of the code consistent.
By using the eslint-plugin-prettier formatting issues will be highlighted as a lint error.
These errors will be auto fixed by running "lint --fix" (which can be done automatically on file save)

* [Prettier] Add "prettier" - Fix formatting

src/components/atoms/ThreeStateCheckbox.tsx

@@ -19,8 +19,8 @@ function nextState(state: CheckState): CheckState {
 }
 
 export interface ThreeStateCheckboxProps extends Omit<CheckboxProps, 'checked' | 'onChange'> {
-    checked?: boolean | undefined | null
-    onChange?: (checked: boolean | undefined | null) => void
+    checked?: boolean | undefined | null;
+    onChange?: (checked: boolean | undefined | null) => void;
 }
 
 /**