fix: select placeholder z-index

[bitpshr]

Type: bug

The following has been addressed in the PR:

• [x] There is a related issue
• [x] All code matches the style guide
• [ ] Unit tests are included in the PR
• [x] For new widgets, an entry has been added to the .dojorc
• [x] For new widgets, theme.variant() is added to the root domnode
• [x] Any widget variant uses theme.compose like this
• [x] WidgetProperties are exported

Description:

This pull request fixes Select placeholder z-index styling using the Material theme.

Resolves #1729

[vercel[bot]]

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dojo.widgets – ./

🔍 Inspect: https://vercel.com/dojo/dojo.widgets/BDzGAaeFJqsZs99hoMZFju4catiV
✅ Preview: https://dojowidgets-git-fork-bitpshr-fix-select-placeholder-z-faf235.vercel.app

widget-test-docs – ./

🔍 Inspect: https://vercel.com/dojo/widget-test-docs/2Jh6edp4r5GJFco6CQsXRccfEMin
✅ Preview: https://widget-test-do-git-fork-bitpshr-fix-select-placeholder-z-c89d14.vercel.app

[codecov[bot]]

Codecov Report

Merging #1738 (a3e3db5) into master (7fbe4e9) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1738   +/-   ##
=======================================
  Coverage   90.24%   90.24%           
=======================================
  Files          94       94           
  Lines        5116     5116           
  Branches     1392     1392           
=======================================
  Hits         4617     4617           
  Misses        244      244           
  Partials      255      255           

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