Update unit selection UX to highlight common and recently-used units and make it easier to find and select units

[grochocki]

Problem Statement
Unit converters in Calculator support a large set of units, which can make it difficult to quickly access common or frequently-used units.

Evidence or User Insights
There is some discussion in #382 regarding having a smaller set of units by default with an option to turn on more in settings. Numerous issues (#108, #167, #271, #325, #275, #515) discuss expanding the set of units or converters further, which improves converters overall, but makes this particular UX problem worse.

Anecdotally, it is clear to see some scenarios where we show common units mixed in with uncommon units:

Proposal
Update the unit selection UX to highlight common and recently-used units and make it easier to find and select units.

Goals

• Users can quickly select commonly-used units
• Users can quickly select recently-used units
• Users can easily find less commonly-used units

Non-Goals

• Determining which converters/units to include is out-of-scope (covered by Converter should integrate (or reuse) online unit conversions (Bing or similar services) to increase category/unit coverage #515)
• Redesign any other parts of converter navigation or UX

Low-Fidelity Concept
One approach might be an AutoSuggestBox/ComboBox hybrid experience. This would allow us to highlight the most common units as well as recently used units, while also allowing users to search-to-filter/select any unit.

_{Instead of People/Places/Things, we could highlight common/recently used units}

Requested Assignment
I'm just suggesting this idea. I don't want to implement it.

[ghost]

This pitch looks like it has everything it needs for review. In the meantime, we'll keep this idea open for discussion so the community has the chance to provide feedback. Check out our New Feedback Process for more info on the user-centered process we follow for new feature development.

[MicrosoftIssueBot]

This is your friendly Microsoft Issue Bot. I've seen this issue come in and have gone to tell a human about it.

[ghost]

We reviewed the pitch and would love to explore this idea further! The pitch is a great start, but there are still some open questions. I am moving this issue into planning to iron out some of those details and I created calculator-specs/unitSelection to track progress. A human will follow up with some feedback on your pitch shortly. Keep in mind that not all ideas that make it into the planning phase are guaranteed to make it to release. For more information on next steps, check out our spec workflow.