feat(xl-multi-column): migrate columns onto the container block API

[nperez0111]

Part 2 of 3 of the container blocks stack (1: core API, 2: multi-column migration ← you are here, 3: docs & examples). Stacked on #2997.

What this does

Migrates @blocknote/xl-multi-column from hand-written ProseMirror nodes onto the container block API from the previous PR, and deletes the legacy compatibility shims that PR carried for it.

• column / columnList are now regular createBlockSpec container blocks (pm-nodes/Column.ts and pm-nodes/ColumnList.ts deleted):
  • columnList: children: { allow: ["column"], min: 2, whenEmptied: "unwrap" }
  • column: placement: "containerOnly", so it can only ever live inside a columnList
  • both meta.draggable: false, matching the previous side-menu behavior
• Column keyboard behavior, repair (collapsing emptied columns, unwrapping single-column lists), drag & drop, and HTML round-trips now come entirely from the generic container machinery; the bespoke column code paths in core are gone.
• Core cleanup: fixColumnList.ts deleted along with every // Legacy shim from the previous PR (blockToNode, internal HTML serializer, UniqueID types, Exporter.isContainerBlock, containerUI, fragmentToBlocks, fixContainer).
• Column resizing moves fully into ColumnResizeExtension (widths are no longer a schema prop concern of core).

Behavior notes

• Conversion snapshots change shape slightly (attribute order, data-children-of markers on the children hosts, data-width now handled by the resize extension).
• insertBlocks with a partial columnList (missing columns/children) now auto-fills from the container config instead of throwing, matching every other container block.

Testing

• fixColumnLists.test.ts → fixContainer.test.ts (same scenarios against the generic repair), new enter.test.ts and columnResize.test.ts.
• Multi-column e2e suite (tests/src/end-to-end/multicolumn) extended for the migrated behavior; full suite green.

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