Fix getSaveWarnings in Free Response widget

[aag]

Summary:

This commit changes the getSaveWarnings() function definition in the
Free Response widget editor to be defined using public class field syntax of
the public method syntax. Previously, this wasn't being bound
properly, so the function was unable to read this.props. Additionally,
the Free Response widget is unhidden, so that it will appear in the widgets
dropdown in the exercise editor.

The commit also adds some more type annotations and changes all methods
on the widget editor to be defined in the same way.

Issue: https://khanacademy.atlassian.net/browse/AX-1005

Test plan:

• Load the PR's Perseus bundle in local webapp.

• Go to the exercise editor in devadmin and verify that there is a "Free Response"
  option in the "Add a widget..." dropdown.
  

• Add a Free Response widget in the exercise editor and verify that it
  renders properly.
  

[github-actions]

Size Change: -5 B (0%)

Total Size: 470 kB

Filename	Size	Change
packages/perseus-editor/dist/es/index.js	89.5 kB	-6 B (-0.01%)
packages/perseus/dist/es/index.js	201 kB	+1 B (0%)

ℹ️ View Unchanged

Filename	Size
packages/kas/dist/es/index.js	20.7 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	98.6 kB
packages/math-input/dist/es/strings.js	1.61 kB
packages/perseus-core/dist/es/index.js	19.5 kB
packages/perseus-linter/dist/es/index.js	7.05 kB
packages/perseus-score/dist/es/index.js	9.22 kB
packages/perseus-utils/dist/es/index.js	403 B
packages/perseus/dist/es/strings.js	7.54 kB
packages/pure-markdown/dist/es/index.js	1.22 kB
packages/simple-markdown/dist/es/index.js	6.72 kB

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[github-actions]

npm Snapshot: Published

Good news!! We've packaged up the latest commit from this PR (275e90b) and published it to npm. You
can install it using the tag PR2531.

Example:

pnpm add @khanacademy/perseus@PR2531

If you are working in Khan Academy's webapp, you can run the below command.
If this is for backend code, change the static flag (-s) from y/yes to n/no.

./dev/tools/bump_perseus_version.js -s y -t PR2531

[jeremywiebe]

Sorry. This is old code that uses refs alot. I'm unclear why this was broken because we call getSaveWarnings on the widget editor ref. Anyways, glad you figured it out.