MAINT: move FeatureData[Importance] type and formats to q2-types

[VinzentRisch]

Problem

q2-sample-classifier currently owns FeatureData[Importance], its TSV format, directory format, and DataFrame/Metadata transformers. This is a broadly useful FeatureData variant, and keeping it in q2-sample-classifier means other plugins need to depend on q2-sample-classifier just to use the semantic type.

This came up in qiime2/q2-feature-table#357, where q2-feature-table needs to emit core scores as FeatureData[Importance] artifacts.

Proposed change

Move the shared type and formats to q2-types as tracked in qiime2/q2-types#389. Then q2-sample-classifier should:

• import Importance from q2_types.feature_data
• keep using FeatureData[Importance] in its action signatures
• remove local Importance type, format, artifact registration, and transformers

After this, q2-sample-classifier remains a consumer of FeatureData[Importance], while q2-types owns the common semantic type and artifact format.