Add a new ComboBox component

[sarahmonster]

Fixes #138.

Still on my to-do list here:

• [x] absolutely position drop-down (this behaved unpredictably when I first tried)
• [x] add search & caret icons (relies on a fix for #84)
• [x] set size when # of options is less than 7 (avoid empty space!)
• [x] figure out how to add actions to the action icon (iconbutton, possibly!)
• [x] implement autocomplete as an optional prop, so you can also use the component as a select
• [x] write docs (could be done as part of a follow-up PR if needed)

It'll also require @tofumatt's help to make it functional, and we'll want to work on a secondary PR to make the autocomplete/searching bit work as well.

Comboboxes:

• https://reacttraining.com/reach-ui/combobox/
• https://www.lightningdesignsystem.com/components/combobox/
• https://vuetifyjs.com/en/components/combobox/
• https://polaris.shopify.com/components/forms/autocomplete#navigation
• https://developer.apple.com/design/human-interface-guidelines/macos/fields-and-labels/combo-boxes/

Selects:

• https://www.carbondesignsystem.com/components/select/
• https://design-system.service.gov.uk/components/select/
• https://material-components.github.io/material-components-web-catalog/#/component/select

Technical/implementation notes:

• https://www.w3.org/TR/wai-aria-practices-1.2/#combobox
• https://adamsilver.io/articles/building-an-accessible-autocomplete-control/

[sarahmonster]

I'm working on the docs for this here: https://www.notion.so/octopusthink/Combobox-80fa2fd9ce4b464ab7ced7757297543f (still in progress!)

There are two key components we'll want to make a Combobox really robust:

• autocompletion (ie, when you type, the result list should filter)
• "add new" as the final option in the list (this should be optional and obviously will need to hook up to a callback)

And a few things we may want to fix in this PR, but I don't know how:

• the magic numbers used to set height if the number of options is less than seven is super fragile
• tests! I didn't write any.

I think those can wait for a follow-on PR, in the interests of keeping things simple, because we can make use of the combobox without those functionalities. Also, I'll probably need (a lot of!) help implementing those, since there's lots of complexity there.

My tests are failing fairly enthusiastically here. I wanted to start writing tests but I'm a bit confused that it looks like some unrelated tests are failing, and I can't seem to/am not sure how to generate snapshots. @tofumatt when you have a moment I'd love your help sussing out what's going on!