Move the `Array.prototype.sort` `comparefn` test away from /es5/

https://kangax.github.io/compat-table/es5/#test-Array_methods_Array.prototype.sort:_compareFn_must_be_function_or_undefined

This test has nothing to do with ES5. It’s a formerly undefined behavior that only [very recently](https://github.com/tc39/ecma262/commit/564b30b3b1bbba3772a2ebeb58208043713d64fe) (years after ES5 was published) got standardized. If anything, it’s an ES2018 feature.

Should this be moved from `…/es5/` to another page (e.g. `…/es2018/`)?

Start with the linked Array.prototype.sort comparefn entry in the compat-table ES5 page and review the referenced ECMAScript commit. Determine the appropriate edition page, then move the test so it is no longer categorized under ES5 and verify the entry remains represented in the compatibility table.

Written by the indexing model from the issue text.