Include vendor styles for WordPress selectors

[dustin-jw]

The theme check plugin can be used to validate themes against a test suite to report any issues with the theme.

We get recommendations to add styles for the following CSS classes:

RECOMMENDED .wp-caption css class is recommended in your theme css.
RECOMMENDED .wp-caption-text css class is recommended in your theme css.
RECOMMENDED .sticky css class is recommended in your theme css.
RECOMMENDED .gallery-caption css class is recommended in your theme css.
RECOMMENDED .bypostauthor css class is recommended in your theme css.
RECOMMENDED .alignright css class is recommended in your theme css.
RECOMMENDED .alignleft css class is recommended in your theme css.
RECOMMENDED .aligncenter css class is recommended in your theme css.
INFO .screen-reader-text CSS class is generated by WordPress and needs to be styled with your theme CSS.

We should add these to a vendors/_wordpress.scss partial with reasonable default styles, possibly looking at other themes to see what they set for those classes.