editorResourcePath.cs

using System;
using System.IO;
using System.Linq;
using UnityEditor;
using UnityEngine.UIElements;

class PathApi {
    public string MonoPath;

    public string Dir() {
        return Path.GetDirectoryName(MonoPath).Replace('\\', '/');
    }

    public VisualTreeAsset Uxml(string name) {
        return Load<VisualTreeAsset>(name, "uxml");
    }

    public StyleSheet Uss(string name) {
        return Load<StyleSheet>(name, "uss");
    }

    T Load<T>(string name, string ext) where T : UnityEngine.Object {
        var p = Dir();
        if (name.Contains("../")) {
            var ups = name.Split(new[] {"../"}, StringSplitOptions.RemoveEmptyEntries);
            var arr = p.Split(new[] {"/"}, StringSplitOptions.RemoveEmptyEntries);
            p = arr.Take(arr.Length - ups.Length).Aggregate((a, b) => $"{a}/{b}");
            p = $"{p}/{ups.Last()}.{ext}";
            var res = AssetDatabase.LoadAssetAtPath<T>(p);
            Asr.IsTrue(res, p);
            return res;
        }
        else {
            p = $"{p}/{name}.{ext}";
            var res = AssetDatabase.LoadAssetAtPath<T>(p);
            Asr.IsTrue(res, p);
            return res;
        }
    }
}