Select: Add --invalid variant to select

[williamernest]

Add the --invalid variant to the select.

This includes adding styles for the --invalid class to match text field, as well as adding support for a --required class (and behavior) for both the native and non-native select.

[kfranqueiro]

FYI #291 also discusses this. Might want to see if there are any relevant details here that you haven't already considered.