Remove add constant method

[tibisabau]

Description

Removed the FieldSet.add_constant() method from Parcels v4 as it is no longer necessary without JIT compilation. In v4, kernel functions can directly inherit variables from their enclosing scope, eliminating the need for an explicit method to add constants to the fieldset.

The implementation maintains backward compatibility for internal use by keeping the constants dictionary and updating internal kernel initialization code to use direct dictionary assignment (fieldset.constants[key] = value) instead of the removed method. Warning messages were updated to guide users toward the new approach.

Closes #2337.

The code in this pull request was generated by GitHub Copilot with the Claude Sonnet 4.5 model.

Checklist if Applicable

• The tests passed – pytest tests
• Linting passed
• Documentation has been added
• CHANGELOG.md has been updated

[erikvansebille]

Thanks @tibisabau, for this very nice and clean implementation. Great that you're helping out with development of Parcels! Once we have green ticks on the CI, I think this is ready to be merged