Add more flexbox utilities

[sebnitu]

Problem

With the addition of flex-gap and gap utilities, it's a good idea to start adding the rest of the missing flex utilities.

Solution

This PR introduces a few more flexbox utilities along with order, grow and shrink variables to better control the variant output. New utilities include:

• flex-direction-[key] - Applies a flex direction property with the provided key as the value.
• flex-wrap-reverse - Applies the flex-wrap property with the value of wrap-reverse.
• flex-order-[key] - Applies the order property with the provided key as the value.
• flex-basis-[key] - Applies the flex-basis property with the provided key (0 || auto) as the value.
• flex-self-[key] - Applies the align-self property with the provided key as the value.

Fixes #375