Should flag outer function even if inner function uses `this`

[andersk]

Currently no violations are flagged for function f() { function g() { return this; } }. We should fix this to const f = () => { function g() { return this; } };.

[JamieMason]

I'm out of time for today but you're right, I've added a failing test at https://github.com/JamieMason/eslint-plugin-prefer-arrow-functions/compare/issue/39?expand=1.

My first hunch is that containsThis needs to check that its closest ancestor which is a function is the node being visited and not another inner function.

eslint-plugin-prefer-arrow-functions/src/guard.ts

Lines 92 to 94 in c95b889

	containsThis(node: TSESTree.Node): boolean {
	return this.containsToken('Keyword', 'this', node);
	}