`LIKE` escape sanity:

* Deprecate `like_escape()`
* Add a method to `$wpdb`, `->esc_like()`, and add unit tests

`$wpdb::esc_like()` is not used yet. As such, many unit tests will throw `Unexpected deprecated notice for like_escape`. Subsequent commits will alleviate this.

Props miqrogroove.
See #10041.

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