Add visible label and ARIA label to Dropdown widget

[daniellewhyte]

Summary:

This commit adds two new fields to the Dropdown widget: ariaLabel and visibleLabel.

Issue: LIT-1424

Test plan:

[github-actions]

npm Snapshot: Published

Good news!! We've packaged up the latest commit from this PR (e6366c8) and published it to npm. You
can install it using the tag PR1845.

Example:

yarn add @khanacademy/perseus@PR1845

If you are working in Khan Academy's webapp, you can run:

./dev/tools/bump_perseus_version.sh -t PR1845

[github-actions]

Size Change: +336 B (+0.03%)

Total Size: 1.29 MB

Filename	Size	Change
packages/perseus-editor/dist/es/index.js	698 kB	+114 B (+0.02%)
packages/perseus/dist/es/index.js	425 kB	+222 B (+0.05%)

ℹ️ View Unchanged

Filename	Size
packages/kas/dist/es/index.js	39 kB
packages/keypad-context/dist/es/index.js	760 B
packages/kmath/dist/es/index.js	4.27 kB
packages/math-input/dist/es/index.js	77.9 kB
packages/math-input/dist/es/strings.js	1.79 kB
packages/perseus-core/dist/es/index.js	1.48 kB
packages/perseus-linter/dist/es/index.js	22.2 kB
packages/perseus/dist/es/strings.js	3.57 kB
packages/pure-markdown/dist/es/index.js	3.66 kB
packages/simple-markdown/dist/es/index.js	12.5 kB

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