Enforce newlines at end of files

Trying to upload a CL with missing newlines at the end of files fails with the following warning:

These files should end in one (and only one) newline character:

Update the ESLint config to let the linter fix these errors.

Change-Id: Ia858b5864a1dc43144e0c6d81dedbf46376c2b8d
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1897679
Commit-Queue: Tim van der Lippe <[email protected]>
Commit-Queue: Yang Guo <[email protected]>
Auto-Submit: Tim van der Lippe <[email protected]>
Reviewed-by: Yang Guo <[email protected]>
diff --git a/front_end/externs.js b/front_end/externs.js
index 7af8e06..b4d6ccf 100644
--- a/front_end/externs.js
+++ b/front_end/externs.js
@@ -1532,4 +1532,4 @@
    */
   close() {
   }
-}
\ No newline at end of file
+}