Add GeometryViewer as oEmbed provider (#859)

GeometryViewer (https://geometryviewer.com) is a free online 3D model
viewer supporting STL, OBJ, GLTF, GLB, and 3MF formats with WebXR AR.
Shareable model links at /v/TOKEN.ext expose oEmbed discovery tags and
return rich iframe embeds via the /oembed endpoint.

Co-authored-by: Christopher Howard <modeldirectory.org@gmail.com>

Author: PrintShaper

providers/GeometryViewer.yml

@@ -0,0 +1,12 @@
+---
+- provider_name: GeometryViewer
+  provider_url: https://geometryviewer.com
+  endpoints:
+  - schemes:
+    - https://geometryviewer.com/v/*
+    - https://www.geometryviewer.com/v/*
+    url: https://geometryviewer.com/oembed
+    example_urls:
+    - https://geometryviewer.com/oembed?url=https%3A%2F%2Fgeometryviewer.com%2Fv%2Fdemo.stl&format=json
+    discovery: true
+...