Introduce `woocommerce_product_is_on_sale` filter

This filter is useful for extensions like Measurement Price Calculator
which alter the product price quite a bit, but would still like the
sale badge to appear :)

Current workaround is to use three filters ( `woocommerce_get_price`,
`woocommerce_get_regular_price`, and `woocommerce_get_sale_price` )
which most extension may be using already, but not all need/want to
alter these prices.