Adding alt tag to image widget on zoom (#1942)

## Summary:
This is a simple PR to ensure that we're retaining any pre-existing image alt tags when the user zooms in on an image. 

While I created a storybook view for testing this work, I have removed it before opening up the PR for review as I am unsure about potential image copyright issues. 

Issue: A11Y-41

## Test plan:
- manual test with storybook 
- manual test in webapp with PR

Author: SonicScrewdriver

Reviewers: mark-fitzgerald, catandthemachines

Required Reviewers:

Approved By: mark-fitzgerald, catandthemachines

Checks: ✅ gerald, ✅ Cypress (ubuntu-latest, 20.x), ✅ Publish npm snapshot (ubuntu-latest, 20.x), ✅ Check builds for changes in size (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), ✅ gerald, ✅ Publish npm snapshot (ubuntu-latest, 20.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), ✅ Cypress (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ Check builds for changes in size (ubuntu-latest, 20.x), ✅ Publish Storybook to Chromatic (ubuntu-latest, 20.x), ✅ gerald

Pull Request URL: #1942

.changeset/hip-pandas-argue.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/perseus": patch
+---
+
+Ensure that zoomed-in images retain alt text

packages/perseus/src/zoom.ts

@@ -364,6 +364,7 @@ Zoom.prototype.zoomImage = function () {
     }.bind(this);
 
     img.src = this._targetImage.src;
+    img.alt = this._targetImage.alt;
 
     this.$zoomedImage = $zoomedImage;
 };