chore: generate codemirror languages from linguist-languages package (#38624)

Replace the previous main-branch fetch with data from npm package
[`linguist-languages`](https://github.com/ikatyang-collab/linguist-languages)
and only run generate targets when actually needed. The removed
`fileFilters` were leftovers from the nolyfill removal.

assets/codemirror-languages.json

@@ -453,7 +453,6 @@
       ".htmlhintrc",
       ".imgbotconfig",
       ".nycrc",
-      ".secrets.baseline",
       ".tern-config",
       ".tern-project",
       ".watchmanconfig",