This PR adds the missing methods for the `align-self` css property. The
`align_self` field existed on the `StyleRefinement` struct but was
inaccessible using the `styled` trait.

Release Notes:

- Adds `self_start`, `self_end`, `self_center`, `self_flex_start`,
`self_flex_end`, `self_baseline`, and `self_stretch` methods to the
`trait Styled`