Support specifying lsp binary

[rubin55]

I'm running my own svelte LSP binary (packaged in Arch Linux extras as svelte-language-server, and a git version in the AUR). The binary is /usr/bin/svelteserver. Can you add a configuration option so you can specify the svelte-language-server binary so it does not download it?

[rubin55]

For posterity, I fixed this by adding the following to settings.json:

"languages": {
  "Svelte": {
    "language_servers": ["svelte-language-server"]
  }
}

"lsp": {
  "svelte-language-server": {
    "binary": {
      "path": "/usr/bin/svelteserver",
      "arguments": ["--stdio"]
    }
  }
}

[gyzerok]

I've made a PR about a different issue, but had some fun doing it. Will look into this if the other one gets accepted.