Add multiselect combobox example with tokens

[smhigley]

Related to this ARIA issue: w3c/aria#689

As discussed in the 9/3/2020 ARIA meeting, it would be helpful to have an APG example of a multiselect combobox that implements tokens for selected items.

One possible example would be a combobox for adding people to an email "To" line.

refs:
https://openui5.hana.ondemand.com/#/entity/sap.m.MultiInput/sample/sap.m.sample.MultiInputMaxTokens
https://joshuajohnson.co.uk/Choices/
https://developer.apple.com/design/human-interface-guidelines/macos/fields-and-labels/token-fields/

[brennanyoung]

FWIW tokens are implemented in an APG example (2) here
https://w3c.github.io/aria-practices/examples/grid/LayoutGrids.html

Would be good if the combobox example is consistent with this one. (But why are they using tabindex="0" on an <a> ???)