Moves the four bulk format-conversion handlers out of main.cpp:
--convert-m2-batch (M2 → WOM)
--convert-wmo-batch (WMO → WOB)
--convert-blp-batch (BLP → PNG)
--convert-dbc-batch (DBC → JSON)
These all share the same pattern: walk srcDir recursively for
files of the input extension and fan out to the single-file
--convert-* counterpart via subprocess (preserving the existing
per-file logic as the source of truth, no duplication).
Single-file converters (--convert-m2, --convert-wmo, etc.) and
the --migrate-* meta-commands still live in main.cpp; they're
in dedicated argv-rescan loops that need a different extraction
approach.
main.cpp drops 18,396 → 18,198 lines (-198). Behavior verified
by re-running --convert-blp-batch with a missing directory and
confirming the same error message.
