Improve Blame Page rendering performance

Supercedes: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/73739
Follow-up on: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/73173

This change adds auto content-visibility to line numbers on Blame Page.
That significantly reduces rendering time for the page.

Changelog: other

app/assets/stylesheets/framework/files.scss

@@ -227,7 +227,7 @@
       // IMPORTANT PERFORMANCE OPTIMIZATION
       //
       // When viewinng a blame with many commits a lot of content is rendered on the page.
-      // Two selectors below ensure that we only render what is visible to the user, thus reducing TBT in the browser.
+      // content-visibility rules below ensure that we only render what is visible to the user, thus reducing TBT in the browser.
       .commit {
         content-visibility: auto;
         contain-intrinsic-size: 1px 3em;
@@ -237,6 +237,10 @@
         content-visibility: auto;
         contain-intrinsic-size: 1px 1.1875rem;
       }
+
+      .line-numbers {
+        content-visibility: auto;
+      }
     }
 
     &.logs {