This PR extracts Deno support into an extension and removes the built-in
Deno support from Zed.

When using the Deno extension, you'll want to add the following to your
settings to disable the built-in TypeScript and ESLint language servers
so that they don't conflict with Deno's functionality:

```json
{
  "languages": {
    "TypeScript": {
      "language_servers": ["deno", "!typescript-language-server", "!eslint", "..."]
    },
    "TSX": {
      "language_servers": ["deno", "!typescript-language-server", "!eslint", "..."]
    }
  }
}

```

Release Notes:

- Removed built-in support for Deno, in favor of making it available as
an extension.