Re-add extensions as top level dir

extensions/ohif-vtk-extension/.gitignore

@@ -0,0 +1,26 @@
+
+# See https://help.github.com/ignore-files/ for more about ignoring files.
+
+# dependencies
+node_modules
+
+# builds
+build
+dist
+.rpt2_cache
+
+# misc
+.DS_Store
+.env
+.env.local
+.env.development.local
+.env.test.local
+.env.production.local
+
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+.idea
+
+yalc.lock
+.yalc