Convert MultiSelect and SingleSelect to functional components (#2370)

## Summary:
Convert MultiSelect and SingleSelect to functional components to support the validation work to support LabeledField.

I was trying to add in the validation logic without refactoring too much since we might replace these components with Combobox. It was more complicated to account for different cases so I am refactoring it first to make it easier!

Issue: WB-1782

## Test plan:
- SingleSelect continues to work as expected
- MultiSelect continues to work as expected

(would appreciate help with extra testing in case I missed something. These components were a bit more complicated to convert to FC!)

Author: beaesguerra

Reviewers: beaesguerra, jandrade

Required Reviewers:

Approved By: jandrade

Checks: ✅ Chromatic - Get results on regular PRs (ubuntu-latest, 20.x), ✅ Test / Test (ubuntu-latest, 20.x, 2/2), ✅ Lint / Lint (ubuntu-latest, 20.x), ✅ Test / Test (ubuntu-latest, 20.x, 1/2), ✅ Check build sizes (ubuntu-latest, 20.x), ✅ Chromatic - Build on regular PRs / chromatic (ubuntu-latest, 20.x), ⏭️  Chromatic - Skip on Release PR (changesets), ✅ Publish npm snapshot (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ gerald, ✅ Prime node_modules cache for primary configuration (ubuntu-latest, 20.x), ⏭️  dependabot

Pull Request URL: #2370

.changeset/mean-sheep-shout.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/wonder-blocks-dropdown": patch
+---
+
+Update SingleSelect and MultiSelect to functional components