only-arrow-functions: Allow functions that use 'this' somewhere in the body

[ghost]

PR checklist

• Addresses an existing issue: #0000
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

What changes did you make?

Changed only-arrow-functions to allow a non-arrow function if this appears somewhere in its body. These cannot be converted to arrow functions.
There is already an exception for this as an explicit parameter, but often it comes from a contextual type, as in:

function f(g: (this: number) => void) { g.call(0); }
f(function() { console.log(this) });

[enhancement] only-arrow-functions allow functions that use this in the body

[nchen63]

lgtm, thanks!