fix(#682): Properties -> add style attr function

[srghma]

No description provided.

[thomashoneyman]

@srghma I'm on board with this change. Historically Halogen has omitted the property so as to encourage use of typed css via purescript-halogen-css, but I don't think it's necessary to enforce that via omission.

Still, it is good for folks to know they can use typed CSS if they would like to. Can you add a documentation comment like the one below to this property?

-- | Sets the `style` attribute to the specified string.
-- |
-- | ```purs
-- | ... [ style "height: 50px;" ]
-- | ```
-- |
-- | If you prefer to use typed CSS for this attribute, you can use the purescript-halogen-css library:
-- | https://github.com/purescript-halogen/purescript-halogen-css

@garyb let me know if you'd prefer not to mention halogen-css here; otherwise I'm happy to merge with this comment.