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.
Adds `make generate-codemirror-languages` to the npm group's
`postUpgradeTasks` in `renovate.json5`, so renovate regenerates
`assets/codemirror-languages.json` whenever `@codemirror/language-data`
(or any npm dep) updates — mirroring the existing `make svg` handling.
Also reformats the `fileFilters` arrays multi-line and regenerates the
asset to pick up current upstream linguist languages.