make: rename ApplyModifiersState_Define to Expr_Define

The type name ApplyModifiersState was only intended as a working draft,
its name is too long and its scope a little too narrow.

Applying the modifiers is the main part of evaluating a variable
expression, and the scope of that type will be extended to parsing the
name of the expression as well.  This will hopefully reduce the number
of parameters, which is currently at 14.

No functional change.

(rillig)