Selector: Backport jQuery selection context logic to selector-native

Selector: Backport jQuery selection context logic to selector-native

This makes:
```js
$div.find("div > *")
```
no longer matching children of `$div`.

Also, leading combinators now work, e.g.:
```js
$div.find( "> *" );
```
returns children of `$div`.

As a result of that, a number of tests are no longer skipped in the
`selector-native` mode.

Also, rename `rcombinators` to `rleadingCombinator`.

Fixes gh-5185
Closes gh-5186
Ref gh-5085