Add `lkml` extension to LookML (#6443)

Add lkml as primary extension for LookML

lib/linguist/languages.yml

@@ -3804,9 +3804,8 @@ LookML:
   codemirror_mime_type: text/x-yaml
   color: "#652B81"
   extensions:
+  - ".lkml"
   - ".lookml"
-  - ".model.lkml"
-  - ".view.lkml"
   tm_scope: source.yaml
   language_id: 211
 LoomScript: