chore: add cem config

Author: bennypowers

.config/cem.yaml

@@ -0,0 +1,14 @@
+sourceControlRootUrl: https://github.com/patternfly/patternfly-elements/tree/main/
+generate:
+  files:
+    - elements/*/*.ts
+    - core/**/*.ts
+  exclude:
+    - '*.test.ts'
+    - '*.spec.ts'
+  output: custom-elements.json
+  noDefaultExcludes: false
+  demoDiscovery:
+    fileGlob: elements/*/demo/*.html
+    urlPattern: elements/(?P<tag>[\\w-]+)/demo/(?P<demo>[\\w-]+).html
+    urlTemplate: https://patternflyelements.org/components/{tag}/{demo}/