Allow to disable default resolver

By default eslint merges the config, making it impossible to use a custom resolver with option without the default resolver node being added.

### Try 1

```js
settings: {
'import/resolver': {
[path.resolve(__dirname, './resolver')]: { /* some options */ },
},
},
```

Then the following resolvers are used:
```js
{
'/home/.../eslint-config/resolver': {},
node: { extensions: [ '.js', '.jsx', '.json' ] }
}
```

### Try 2:

```js
settings: {
'import/resolver': {
[path.resolve(__dirname, './resolver')]: { /* some options */ },
node: false,
},
},
```

Then the following resolvers are used:
```js
{
'/home/.../eslint-config/resolver': {},
node: false,
}
```

And the node resolver is still used, same with null, undefined or other

### Try 3:

I can only disable resolver node by doing this, but I can't pass any options:

```js
settings: {
'import/resolver': path.resolve(__dirname, './resolver'),
},
```

---

I propose as a solution not to use a resolver if its configuration is equal to false.

Start by tracing how the `settings['import/resolver']` configuration is interpreted, focusing on the custom resolver options and the default `node` resolver. Done means a resolver configured as `false` is not used while other configured resolvers still receive their options; add regression coverage for this behavior.

Written by the indexing model from the issue text.