Add support for the BEX extended string syntax

Boom added an alternate method for overriding Dehacked strings, using the [STRINGS] section with special symbolic names for each of the strings that can possibly be replaced.

This format is used in the Freedoom DEHACKED lump (deliberately, so that the Freedoom WAD does not include the original text to be replaced). As we want to have support for the Freedoom IWADs with Chocolate Doom, add support for the BEX [STRINGS] section, but protected by a magic comment that must be included in Dehacked/BEX files that use it.