Make xESMF a required dependency

[tomvothecoder]

Description

• Closes #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

• [x] 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
• [x] Make xESMF a required dependency in the conda-forge feedstock (https://github.com/conda-forge/xcdat-feedstock/issues/27)
• [x] Remove any code that detects whether or not xESMF exists in the env since it should always exist in the env
• [x] Update getting-started documentation to remove mentions of xESMF as an optional dependency

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] 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
• [x] 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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

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