Add all alphabet icons (#669)

Add all icons for latin alphabet inputs to keypad
LC-1143
<img width="1792" alt="Screenshot 2023-08-15 at 2 59 14 PM" src="https://github.com/Khan/perseus/assets/23404711/c1a4556c-9f24-4c1a-824f-3c9b1b5c9b29">

Author: nedredmond

Reviewers: jeremywiebe, handeyeco, SonicScrewdriver

Required Reviewers:

Approved By: jeremywiebe, handeyeco

Checks: ✅ finish_coverage, ✅ Publish npm snapshot (ubuntu-latest, 16.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 16.x), ✅ Extract i18n strings (ubuntu-latest, 16.x), ✅ Cypress Coverage (ubuntu-latest, 16.x), ✅ Jest Coverage (ubuntu-latest, 16.x), ✅ Check for .changeset file (ubuntu-latest, 16.x), ✅ Check builds for changes in size (ubuntu-latest, 16.x), ✅ gerald, ✅ finish_coverage, ✅ finish_coverage, ✅ finish_coverage, ✅ Publish npm snapshot (ubuntu-latest, 16.x), ✅ Extract i18n strings (ubuntu-latest, 16.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 16.x), ✅ Check for .changeset file (ubuntu-latest, 16.x), ✅ Cypress Coverage (ubuntu-latest, 16.x), ✅ Check builds for changes in size (ubuntu-latest, 16.x), ✅ gerald, ✅ Jest Coverage (ubuntu-latest, 16.x), ✅ Publish npm snapshot (ubuntu-latest, 16.x), ✅ Extract i18n strings (ubuntu-latest, 16.x), ✅ Check builds for changes in size (ubuntu-latest, 16.x), ✅ Cypress Coverage (ubuntu-latest, 16.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 16.x), ✅ Jest Coverage (ubuntu-latest, 16.x), ✅ Check for .changeset file (ubuntu-latest, 16.x), ✅ gerald, ✅ Publish npm snapshot (ubuntu-latest, 16.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 16.x)

Pull Request URL: #669

.changeset/rich-spies-design.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/math-input": patch
+---
+
+Add icons for all letters