Expose selected text and blocks from Selection API

[RomeoApps]

Summary

Adds a small public selection API for the multiblock selection use case discussed in #2410:

• api.selection.getSelectedText() returns native editor selection text when a normal text range is active, or selected-block text for Editor.js cross-block selections.
• api.selection.getSelectedBlocks() returns the selected BlockAPI[].
• Block-selection copy text and selected-block text now share the same sanitized fragment path.
• Native editable text drags are preserved instead of being converted into whole-block selection.

Validation

• git diff --check origin/next..HEAD
• YARN_IGNORE_ENGINES=1 yarn eslint src/components/modules/crossBlockSelection.ts src/components/modules/blockSelection.ts src/components/modules/api/selection.ts test/cypress/tests/selection.cy.ts --quiet
• YARN_IGNORE_ENGINES=1 yarn cypress run --spec test/cypress/tests/selection.cy.ts -> 3 passing

Prior broader validation on this branch also passed yarn lint, yarn build:test, yarn build, full yarn test:e2e, and a live browser probe. I did not rerun the full suite in this PR refresh pass.

Notes

This does not claim a complete replacement for browser-native arbitrary cross-block text selection behavior. It adds the requested programmatic access to selected text/blocks and preserves native editable drags where they already exist.

Refs #2410.