[TypeSpec Validation] Trigger "all" pipeline on changes to common files (#25488)

- Ensures changes are validated on Windows and Linux

Author: mikeharder

eng/pipelines/typespec-validation-all.yml

@@ -4,7 +4,19 @@ trigger:
     - main
     - typespec-next
 
-pr: none
+pr:
+  branches:
+    include:
+    - main
+    - typespec-next
+  paths:
+    include:
+    - .gitattributes
+    - .prettierrc.json
+    - package-lock.json
+    - package.json
+    - tsconfig.json
+    - eng
 
 jobs:
 - job: Validate_All_Specs