refactor: convert <UnitButton> and <UnitIcon> to TSX

[bradenmacdonald]

Description

Small PR to convert <UnitButton> and <UnitIcon> to TypeScript.

Supporting information

Private ref MNG-4670

Testing instructions

Make sure the unit nav bar works:

Best Practices Checklist

We're trying to move away from some deprecated patterns in this codebase. Please
check if your PR meets these recommendations before asking for a review:

• Any new files are using TypeScript (.ts, .tsx).
• Avoid propTypes and defaultProps in any new or modified code.
• Tests should use the helpers in src/testUtils.tsx (specifically initializeMocks)
• Do not add new fields to the Redux state/store. Use React Context to share state among multiple components.
• Use React Query to load data from REST APIs. See any apiHooks.ts in this repo for examples.
• All new i18n messages in messages.ts files have a description for translators to use.
• Avoid using ../ in import paths. To import from parent folders, use @src, e.g. import { initializeMocks } from '@src/testUtils'; instead of from '../../../../testUtils'

[openedx-webhooks]

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[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.82%. Comparing base (8f06263) to head (2164034).

Files with missing lines	Patch %	Lines
...course-sequence/sequence-navigation/UnitButton.tsx	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2649      +/-   ##
==========================================
- Coverage   94.82%   94.82%   -0.01%     
==========================================
  Files        1231     1231              
  Lines       27629    27637       +8     
  Branches     6223     6223              
==========================================
+ Hits        26199    26206       +7     
- Misses       1359     1360       +1     
  Partials       71       71              

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[ChrisChV]

Looks good! It's missing fixing the coverage to merge
👍

• I tested this: I verified that the unit nav bar works
• I read through the code and considered the security, stability and performance implications of the changes.
• I tested that the UI can be used with a keyboard only (tab order, keyboard controls).