fix(structures): correctly check if flags are present (#11404)

* fix(structures): correctly check if flags are present

* chore: isFieldSet type guard

---------

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

packages/structures/src/utils/type-guards.ts

@@ -1,3 +1,32 @@
 export function isIdSet(id: unknown): id is bigint | string {
 	return typeof id === 'string' || typeof id === 'bigint';
 }
+
+interface TypeMap {
+	bigint: bigint;
+	boolean: boolean;
+	function(...args: any[]): unknown;
+	number: number;
+	object: object;
+	string: string;
+	symbol: symbol;
+	undefined: undefined;
+}
+
+export type TypeofType = keyof TypeMap;
+
+function hasProperty<Value extends object, Key extends string>(
+	data: Value,
+	fieldName: Key,
+): data is Record<Key, unknown> & Value {
+	return fieldName in data;
+}
+
+export function isFieldSet<Value extends object, Key extends string, Type extends TypeofType>(
+	data: Value,
+	fieldName: Key,
+	type: Type,
+): data is Record<Key, TypeMap[Type]> & Value {
+	// eslint-disable-next-line valid-typeof
+	return hasProperty(data, fieldName) && typeof data[fieldName] === type;
+}