PR: Make the plugin work with Spyder prereleases, fix tests and simplify setup on CI

[ccordoba12]

• Our tests were failing because the plugin couldn't install Spyder-kernels 3.1.0a1, which is now the minimal version required by Spyder 6.1.0a2.
• To fix that, I added the Spyder-kernels prerelease channels to the conda-forge one if we detect Spyder's version is a prerelease.
• Switch to micromamba instead of conda on CI (for some odd reason conda 25 can't resolve the prerelease conda-forge channels).
• Simplify setup on CI by using env yaml files for micromamba and adding an extra_requires/test entry for pip.
• Add codecov.yml to configure codecov and avoid a failure on PRs due to coverage.
• Depends on PR: Fix passing multiple channels when creating envs/installing packages with the Conda-like backend envs-manager#31.

[codecov]

Codecov Report

❌ Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.60%. Comparing base (e0f38c1) to head (c63a6a1).
⚠️ Report is 45 commits behind head on main.

Files with missing lines	Patch %	Lines
spyder_env_manager/spyder/widgets/main_widget.py	70.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
+ Coverage   76.53%   76.60%   +0.07%     
==========================================
  Files          11       11              
  Lines         963      966       +3     
==========================================
+ Hits          737      740       +3     
  Misses        226      226              

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