[WIP] add TeX support to dropdown

[daniellewhyte]

Summary:

WIP! Just pushing this branch so that Juan can take a look.

Issue: LIT-1425

Test plan:

[github-actions]

npm Snapshot: Published

Good news!! We've packaged up the latest commit from this PR (b439537) and published it to npm. You
can install it using the tag PR1810.

Example:

yarn add @khanacademy/perseus@PR1810

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

./dev/tools/bump_perseus_version.sh -t PR1810

[github-actions]

Size Change: +31 B (0%)

Total Size: 866 kB

Filename	Size	Change
packages/perseus/dist/es/index.js	419 kB	+31 B (+0.01%)

ℹ️ View Unchanged

Filename	Size
packages/kas/dist/es/index.js	38.8 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	77.8 kB
packages/math-input/dist/es/strings.js	1.79 kB
packages/perseus-core/dist/es/index.js	1.48 kB
packages/perseus-editor/dist/es/index.js	280 kB
packages/perseus-linter/dist/es/index.js	22.2 kB
packages/perseus/dist/es/strings.js	3.4 kB
packages/pure-markdown/dist/es/index.js	3.66 kB
packages/simple-markdown/dist/es/index.js	12.4 kB

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[daniellewhyte]

This will be ready for review once this update has been released to WB: Khan/wonder-blocks#2354