ENH: Convert ThicknessMap s4ext to json file

Also set `build_subdirectory` to `.` instead of `inner-build` (the former
is specific to SuperBuild extension).

Author: jcfr

ThicknessMap.json

@@ -0,0 +1,8 @@
+{
+  "$schema": "https://raw.githubusercontent.com/Slicer/Slicer/main/Schemas/slicer-extension-catalog-entry-schema-v1.0.0.json#",
+  "build_dependencies": [],
+  "build_subdirectory": ".",
+  "category": "Shape Analysis",
+  "scm_revision": "master",
+  "scm_url": "https://github.com/fatihtalu/Slicer-ThicknessMap"
+}