Rename files to preserve history

Test\Eslint fail - this is expected.

Change-Id: Ie4a0aaa9698db2b19970eb4477b4dbcf4b76b7dd
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.js b/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts
similarity index 100%
rename from polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.js
rename to polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts