Rename GeomPackPreviewMesh -> GeomPackPreviewMeshThree

The three.js preview's generic name made it unclear next to the VTK/Dual/
Multi/Batch previews. Renamed the node id (schema + both mappings), the
widget file (mesh_preview.js -> mesh_preview_three.js, extension name
geometrypack.meshpreview -> geometrypack.meshpreview.three), and the four
test workflows that reference the node type.

Breaking for external saved workflows that used GeomPackPreviewMesh (node
type no longer exists) -- accepted; the pack's own workflows are updated.

javascript/js/mesh_preview_three.js

@@ -15,10 +15,10 @@ const EXTENSION_FOLDER = (() => {
 console.log("[GeomPack] Loading mesh preview extension...");
 
 app.registerExtension({
-    name: "geometrypack.meshpreview",
+    name: "geometrypack.meshpreview.three",
 
     async beforeRegisterNodeDef(nodeType, nodeData, app) {
-        if (nodeData.name === "GeomPackPreviewMesh") {
+        if (nodeData.name === "GeomPackPreviewMeshThree") {
             console.log("[GeomPack] Registering Preview Mesh node");
 
             const onNodeCreated = nodeType.prototype.onNodeCreated;