MAINT: move Importance formats and type to q2-types

[VinzentRisch]

• Closes MAINT: move FeatureData[Importance] type and formats from q2-sample-classifier #389
• Supports ENH: add action to calculate core score for features q2-feature-table#357

Summary

Moves the shared FeatureData[Importance] type and importance TSV formats into q2-types so downstream plugins can use them without depending on q2-sample-classifier. The _MultiColumnNumericFormat was removed and the logic was moved directly to the ImportanceFormat.

Added

• Type: Importance
• Format classes: ImportanceFormat, ImportanceDirectoryFormat
• Registrations: Importance semantic type, ImportanceFormat, ImportanceDirectoryFormat, FeatureData[Importance] -> ImportanceDirectoryFormat
• Transformers: pd.DataFrame -> ImportanceFormat, ImportanceFormat -> pd.DataFrame, ImportanceFormat -> qiime2.Metadata
• Tests and fixtures for importance format validation, type/format registration, and transformers

This PR was created in parts with Codex 5.5

[ebolyen]

Looks great, thanks @VinzentRisch!