[UnlimitedPolygon] Fix open polygon scoring issues.

[catandthemachines]

Summary:

This PR fixes and adds 3 things to the Unlimited Polygon Experience:

1. Only closed unlimited polygons are scorable.
2. Correct Unlimited Polygons in our editor are closed when rendered (i.e. unlimited polygons that are rendered with exisitng coordinates of 3 or more are closed at start).
3. Added save warnings to editor to notify content creators to close their unlimited polygons before saving.

Issue: LEMS-2682

Test plan:

1. Go to http://localhost:6006/?path=/story/perseuseditor-widgets-interactive-graph--interactive-graph-unlimited-polygon.
2. Notice that the unlimited polygon correct answer is automatically closed on render.
3. Go to http://localhost:6006/?path=/story/perseus-widgets-interactive-graph--unlimited-polygon-with-mafs
4. Draw a polygon and check it when it's open, you should see the score is type invalid
5. Close the polygon and git it again, it should be a valid score.

[github-actions]

npm Snapshot: Published

Good news!! We've packaged up the latest commit from this PR (cf99339) and published it to npm. You
can install it using the tag PR1999.

Example:

yarn add @khanacademy/perseus@PR1999

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

./dev/tools/bump_perseus_version.sh -t PR1999

[github-actions]

Size Change: +261 B (+0.02%)

Total Size: 1.27 MB

Filename	Size	Change
packages/perseus-editor/dist/es/index.js	688 kB	+89 B (+0.01%)
packages/perseus/dist/es/index.js	416 kB	+172 B (+0.04%)

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	78 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	4.12 kB
packages/pure-markdown/dist/es/index.js	3.67 kB
packages/simple-markdown/dist/es/index.js	12.5 kB

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[nishasy]

I like this approach. I like how straightforward it is.