Add add-ffi-var

Author: ospencer

api/env.wit

@@ -40,4 +40,5 @@ request-get-field: function(field-type: field-type, key: string, ident: u32) ->
 get-static-file: function(name: string, ident: u32) -> s32
 db-exec: function(query-type: query-type, name: string, ident: u32) -> s32
 get-ffi-result: function(ptr: u32, ident: u32) -> s32
+add-ffi-var: function(name: string, val: string, ident: u32) -> s32
 return-abort: function(msg: string, file: string, line-num: u32, col-num: u32, ident: u32)