Moves the five single-file format converters (--convert-m2,
--convert-wmo, --convert-dbc-json, --convert-json-dbc,
--convert-blp-png) out of two dedicated post-loop blocks in
main.cpp into a new cli_convert_single.{hpp,cpp} module. The
batch wrappers in cli_convert.cpp keep working — they shell
out to wowee_editor with these flags.
main.cpp shrinks by ~335 lines (10,979 → 10,644). The two
trailing for-loops were dead code after wiring the dispatcher
into the main loop, so they're gone too.
