katex: init at 0.10.2

[mt-caret]

Motivation for this change

Add katex, a program that accepts a subset of TeX math and converts it to HTML.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[alexarice]

nodePackages_10_x version works fine though there is no attribute for the nodePackages_8_x package (I'm assuming there should be one as a load of node v8 files have changed). I don't really know enough about js to review properly and I don't know how hard this would be to merge at this point

[veprbl]

@mt-caret Do you still need katex to be included in nixpkgs?

[prusnak]

Please rework your PR. It now has a merge conflict after PR #89184 has been merged

[mt-caret]

@veprbl @prusnak Apologies for the late response. I'm no longer super interested in including this in nixpkgs, so I'm closing this PR.