feat(javascript): Add missing type to prelude.d.ts
`CustomType` was not defined in the prelude TS definition, but it _is_
used in the `.mjs` files. This fixes the issue by adding the
`CustomType` TS definition.
+export class CustomType {
+ inspect(): string;
+ withFields<K extends keyof this>(fields: { [P in K]: this[P] }): this;
+}
+
export interface ListStatic {
fromArray<T>(array: Array<T>): List<T>;
isList(value: unknown): boolean;
