Move RecipesBase dependency to an extension

[ararslan]

This implements the to-do item noted at the top of src/plots.jl, now that the minimum supported Julia version for this package is 1.10.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.75%. Comparing base (ae3f434) to head (43cdf80).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
ext/PolynomialsRecipesBaseExt.jl	80.00%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #618   +/-   ##
=======================================
  Coverage   76.75%   76.75%           
=======================================
  Files          38       37    -1     
  Lines        4065     4065           
=======================================
  Hits         3120     3120           
  Misses        945      945           

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[jverzani]

Thanks so much!