Alias SGM/SIGMA1, refactor aliasing.

[bkeepers]

Adds SIGMA1 as an alias for the existing SGM constituent. This also refactors constituent definitions to just allow specifying all names as part of the definition instead of creating a separate file that imports the existing definition.

cc openwatersio/tide-database#16

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (aa121ae) to head (8830ad1).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #187   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           11        11           
  Lines          469       470    +1     
  Branches        58        58           
=========================================
+ Hits           469       470    +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.