refactor(combobox): use sub component mod properties (#2375)

* refactor(combobox): use sub component mod properties

- Refactors much of the CSS to use mod properties for its sub components
  Textfield and Picker Button. All WHCM style are now handled within
  those components.

- Includes some additional selectors to fix the border styles around
  the picker button so they match when interacting with the textfield,
  which is in more alignment with the gifs shown in the Guidelines.
  Previously when the textfield was focused for example, the focus
  border color did not extend around the whole combobox because the
  picker button border color was not changing as well.

* docs(combobox): bubble up focus classes

The .is-focused and .is-keyboardFocused classes were not being bubbled
up to the parent .spectrum-Combobox (as they should per the docs), which
was resulting in some slightly different styles.

* docs(combobox): replace deprecated icon class from example markup

Replace renamed icon class, per migration guide docs for Picker button.

* docs(combobox): storybook - add missing invalid class

The PickerButton does not currently accept isInvalid and isValid args,
so these classes needed to be added to the customClasses array instead.
The is-invalid class was missing on the picker button when the invalid
control was set to true.

Author: jawinn