Add Dune (OCaml build system)

[kisaragi-hiu]

Description

Dune is a build system for OCaml, used in a large number of OCaml projects. This PR adds classification and syntax highlighting support for it.

I specified ace_mode: lisp because Dune is an Sexp-based language. I'm guessing that this is appropriate.

Checklist:

• I am adding a new language.
  • The extension filename of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each file name:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A%2F%5Edune%24%2F
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A%2F%5Edune-file%24%2F
        • This one is probably justified as it's used by Dune itself, even if just once.
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A%2F%5Edune-project%24%2F
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A%2F%5Edune-workspace%24%2F
        • "dune-workspace" is the name that has a special meaning in Dune, but it's more common to write it in dune-workspace.dev then explicitly tell Dune to use that file.
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A%2F%5Edune-workspace%5C.dev%24%2F
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • dune
      • dune-project
      • dune-file
      • dune-workspace
      • dune-workspace.dev
    • Sample license(s):
      • reasonml/reason
      • ocaml/dune
      • revery-ui/revery
  • I have included a syntax highlighting grammar: https://github.com/maelvls/vscode-dune
    • ocamllabs/vscode-ocaml-platform might be more suitable in the long run, but it errored out when I tried to add it. It also provides grammars for OCaml itself within the same repository, which makes using it more complicated right now.
  • I have added a color
    • Hex value: #89421e
    • Rationale: This is the main color of the Dune logo.
  • I have updated the heuristics to distinguish my language from others using the same extension.