Support TextMeshPro v4.0.0 #5

In TextMeshPro v4.0.0, the TMP_FontAsset class has been deprecated and replaced by UnityEngine.TextCore.Text.FontAsset.
https://docs.unity3d.com/Packages/com.unity.textmeshpro@4.0/api/TMPro.TMP_FontAsset.html

[Obsolete("TMP_FontAsset has been deprecated. Use FontAsset instead (UnityUpgradable) -> [UnityEngine.TextCoreTextEngineModule] UnityEngine.TextCore.Text.FontAsset", true)]
[ExcludeFromDocs]
public class TMP_FontAsset