refactor(search): extract LegacyIndexer wrapper for BC contract

Move the deprecated helpers (lookupKey, addMetaKeys, renameMetaValue,
getPID, lookup) off Indexer and into a new LegacyIndexer wrapper

refactor(search): extract LegacyIndexer wrapper for BC contract

Move the deprecated helpers (lookupKey, addMetaKeys, renameMetaValue,
getPID, lookup) off Indexer and into a new LegacyIndexer wrapper. The
wrapper also restores the Doku_Indexer return contract (true|string)
around addPage/deletePage/renamePage/clear so plugins using the legacy
API keep working without try/catch.

idx_get_indexer() now returns the LegacyIndexer; getPages stays on
Indexer because plugins call it directly on Indexer instances.

fixes #4645

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