Update the Expression widget to support the 'Show Your Work' hackathon project

[kevinb-khan]

Summary:

This PR makes the following changes:

• adds a 'setValue' method on the Expression widget that can be used to set the value of the input field programmatically
• adds a 'disabled' prop which will be used to disable to widget when its rendered outside a Renderer
• adds a 'noBackground' prop which removes the 'background: white' styling that's present by default

The reason for rendering the Expression widget outside of the Renderer is so that we can access the new 'setValue' method on it.

Issue: None

Test plan:

• install the PR published npm module in webapp and do some manual testing there to ensure that setValue is work as expected
• write some unit tests for this

[github-actions]

npm Snapshot: Published

Good news!! We've packaged up the latest commit from this PR (0f3349d) and published it to npm. You
can install it using the tag PR1768.

Example:

yarn add @khanacademy/perseus@PR1768

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

./dev/tools/bump_perseus_version.sh -t PR1768

[github-actions]

Size Change: +198 B (+0.02%)

Total Size: 858 kB

Filename	Size	Change
packages/perseus/dist/es/index.js	409 kB	+198 B (+0.05%)

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	282 kB
packages/perseus-linter/dist/es/index.js	22.2 kB
packages/perseus/dist/es/strings.js	3.49 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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