Remove wordset

Expressions are essentially worde sets. An expression can be required
or optional. Also tweaked some documentation since word set is now used
as a general term for an expression that can match player input.

Renamed some struct members in expr as well, for clarity.

Moved T_REQUIRED/OPTIONAL from removed file wordset.c to expr.c as
T__REQUIRED/OPTIONAL. These enumerations will be used to interpret
the type of expression when it is being matched to an input.
expr.h: Fix declaration of internal

Was causing an implicit declaration warning.

[ci skip]