`nvim-lspconfig` not detecting `jsonnet_ls` on file-open

Followed guide [here](https://github.com/grafana/jsonnet-language-server/tree/main/editor/vim#jsonnet-language-server-integration-for-vim) for nvim-lspconfig, then installed `jsonnet_ls` using `LspInstall`.

`LspInfo` does recognize the `jsonnet_ls` config, but the filetype is not detected.

Also followed the steps outlined in the [nvim-lspconfig server config doc](https://github.com/neovim/nvim-lspconfig/blob/master/doc/server_configurations.md#jsonnet_ls).

Start by reproducing the nvim-lspconfig setup from the linked Jsonnet language-server Vim guide and the server configuration documentation, using LspInstall and LspInfo. Check why the filetype is not detected when a Jsonnet file opens; done means jsonnet_ls is recognized and starts automatically for the relevant filetype.

Written by the indexing model from the issue text.