[default-layout] Hints mode

[thomax]

This PR adds an open/close-able tray (aka side-car) to default layout and an interface to control what's rendered inside. Also, a new plugin @sanity/studio-hints implements this part and introduces hints which are hopefully useful to Studio developers just starting out.

There's some commented out code in @sanity/studio-hints which will do an appearance shortly.

Type of change (check at least one)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (fix or update to documentation)
• Maintenance
• Other, please describe:

Does this change require a documentation update? (Check one)

• Yes
• No (not yet, this will be a non-public feature for the time being)

Checklist

• I have read the Contributing Guidelines
• The PR title includes a link to the relevant issue
• The PR title is appropriately formatted: [some-package] PR title (#123)
• The code is linted
• The test suite is passing
• Corresponding changes to the documentation have been made

[skogsmaskin]

If @bjoerge's comments are attended to, I approve of this PR.