Add `.envrc` and `flake.lock` extensions

[bpeetz]

I've added the .envrc (used by at least direnv) and the nix flake lock file (flake.lock) to the recognized extensions. I've not added an entry in the changelog, as it seemed to tiny to be included in it (let me know if I should include it.).

• [x] My changes do not contradict the current specification.
• [x] I agree to license my contribution under the licenses indicated in the changed files.

[carmenbianca]

Thanks @bpeetz !