feat: overhaul package registry script runners

[hougesen]

The current implementation assumes that the user always want to use a script runner like npx to run the tool, if a binary isn't found.

This makes sense for some tools, but it has the side effect of downloading external code to the users machine. Downloading extra code should be optional, not the default.

This PR addresses that issue by removing the javascript_runtime value from the config, and adding a new value runners that allows the user to define which package runners should be enabled.

{
  "runners": {
    "bunx": false,
    "deno": false,
    "npx": false,
    "pnpm": false,
    "pipx": false,
    "uv": false
  }
}