Since other people mentioned there's been work there, here is my Emacs config now:

  (use-package janet-ts-mode
    :ensure t
    :defer t
    :vc (:url "https://github.com/sogaiu/janet-ts-mode"
            :rev :newest))

  (use-package ajrepl
    :ensure t
    :defer t
    :vc (:url "https://github.com/sogaiu/ajrepl"
            :rev :newest)
    :hook (janet-ts-mode . ajrepl-interaction-mode))

Also, a fix for treesit auto to not ask for the grammar every time:

  (use-package treesit-auto
    :pin melpa
    :ensure t
    :custom
    (treesit-auto-install 'prompt)
    :config
    (add-to-list 'treesit-load-name-override-list
               '(janet "libtree-sitter-janet-simple" "tree_sitter_janet_simple"))
    (add-to-list 'treesit-language-source-alist
             '(janet-simple . ("https://github.com/sogaiu/tree-sitter-janet-simple")))  
    (treesit-auto-add-to-auto-mode-alist 'all)
    (global-treesit-auto-mode))