Make xESMF a required dependency

[tomvothecoder]

Description

• Closes [Feature]: Make xESMF a required dependency since ESMF supports Windows now #562
• Move xESMF to the "base" section in the conda env yml files
• Remove all code related to _has_xesmf() and move xESMF related imports outside of conditional statements
• Remove conditiondal statements related to xESMF that are no longer needed
• Remove mentions of xESMF being an optional dependency in the getting-started documentation page
• Update tests

Todo

• Test installation on xCDAT with xESMF in a conda env on Windows to ensure it builds and xesmf imports work -- tested on my personal Windows PC and it works
• Make xESMF a required dependency in the conda-forge feedstock (Make xESMF a required dependency conda-forge/xcdat-feedstock#27)
• Remove any code that detects whether or not xESMF exists in the env since it should always exist in the env
• Update getting-started documentation to remove mentions of xESMF as an optional dependency

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

If applicable:

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass with my changes (locally and CI/CD build)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have noted that this is a breaking change for a major release (fix or feature that would cause existing functionality to not work as expected)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (577d7b6) 100.00% compared to head (d0b85ca) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #566   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           15        15           
  Lines         1605      1602    -3     
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- Hits          1605      1602    -3     

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