[Expression] Create expression widget generator for testdata

[nishasy]

Summary:

Create a generator for the Expression widget to make testdata easier to read and write.

Issue: https://khanacademy.atlassian.net/browse/LEMS-3771

Test plan:

pnpm jest

[github-actions]

🗄️ Schema Change: No Changes ✅

[github-actions]

npm Snapshot: Published

Good news!! We've packaged up the latest commit from this PR (c282f07) and published it to npm. You
can install it using the tag PR3092.

Example:

pnpm add @khanacademy/perseus@PR3092

If you are working in Khan Academy's frontend, you can run the below command.

./dev/tools/bump_perseus_version.ts -t PR3092

If you are working in Khan Academy's webapp, you can run the below command.

./dev/tools/bump_perseus_version.js -t PR3092

[github-actions]

🛠️ Item Splitting: No Changes ✅

[nishasy]

Sorry, this one is much longer than the other ones I've done so far

[github-actions]

Size Change: +41 B (+0.01%)

Total Size: 499 kB

Filename	Size	Change
packages/perseus-core/dist/es/index.js	22.7 kB	+80 B (+0.35%)
packages/perseus/dist/es/index.js	202 kB	-39 B (-0.02%)

ℹ️ View Unchanged

Filename	Size
packages/kas/dist/es/index.js	20.8 kB
packages/keypad-context/dist/es/index.js	1 kB
packages/kmath/dist/es/index.js	5.98 kB
packages/math-input/dist/es/index.js	99.2 kB
packages/math-input/dist/es/strings.js	1.61 kB
packages/perseus-core/dist/es/index.item-splitting.js	13.1 kB
packages/perseus-editor/dist/es/index.js	98.2 kB
packages/perseus-linter/dist/es/index.js	8.64 kB
packages/perseus-score/dist/es/index.js	9.2 kB
packages/perseus-utils/dist/es/index.js	403 B
packages/perseus/dist/es/strings.js	7.73 kB
packages/pure-markdown/dist/es/index.js	1.39 kB
packages/simple-markdown/dist/es/index.js	6.72 kB

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