This scripted importer will allow to import files with .lua extension as text assets into Unity.

LuaScriptImporter.cs

using System.IO;
using UnityEditor;
using UnityEditor.AssetImporters;
using UnityEngine;

/// <summary>
/// Scripted importer for files with .lua extension.
/// Will create a text asset from the lua script
/// </summary>
[ScriptedImporter(1, "lua")]
public class LuaScriptImporter : ScriptedImporter
{
    public override void OnImportAsset(AssetImportContext ctx)
    {
        TextAsset textAsset = new TextAsset(File.ReadAllText(ctx.assetPath));
        ctx.AddObjectToAsset("lua script", textAsset);
        ctx.SetMainObject(textAsset);
    }
}

[CustomEditor(typeof(LuaScriptImporter))]
internal class LuaScriptImporterEditor : AssetImporterEditor
{
    protected override bool needsApplyRevert => false;
    public override void OnInspectorGUI(){ }
}