fix: add field casting

src/Field.ts

@@ -1,4 +1,5 @@
 export type Field = {
   name: string;
   db: string;
+  cast?: (value: any) => any;
 };

src/grammar.ts

@@ -30,26 +30,31 @@ export const create_semantics = (
   const s = grammar.createSemantics();
   s.addAttribute("asMongo", {
     FieldExpr(name, operator, value) {
+      // Valid field
+      const token = name.asMongo;
+      const field_def = valid_fields.find((f) => f.name == token);
+      if (!field_def) {
+        throw new FaableQLError(`${token} is not a valid field to query`);
+      }
+
       const ops = {
         ":": "$eq",
         "!:": "$ne",
       };
       const op = ops[operator.asMongo];
-      let val;
+
+      const set_value = field_def.cast
+        ? field_def.cast(value.asMongo)
+        : value.asMongo;
+      let mdb_operator;
       if (op == "$eq") {
-        val = value.asMongo;
+        mdb_operator = set_value;
       } else {
-        val = { [op]: value.asMongo };
+        mdb_operator = { [op]: set_value };
       }
 
-      // Valid field
-      const token = name.asMongo;
-      const field_def = valid_fields.find((f) => f.name == token);
-      if (!field_def) {
-        throw new FaableQLError(`${token} is not a valid field to query`);
-      }
       const fieldname = field_def.db;
-      return { [fieldname]: val };
+      return { [fieldname]: mdb_operator };
     },
     TextExpr(text) {
       return {