Add search to SplitView

[interactivellama]

This is possibly for Einstein Insights.

Current page headers with search on Platform look like this:

@kwilloughby-sf For visibility.

[kwilloughby-sf]

Search should follow the In-Context Search Guidelines (https://www.lightningdesignsystem.com/guidelines/search/in-context/). For example, the user shouldn't have to press enter to start the search and things should be highlighted.