text: Title: Implement FloatConverter
  Body: Implement `FloatConverter` along the lines of [`IntConverter`](https://falcon.readthedocs.io/en/stable/api/routing.html#falcon.routing.IntConverter). Draw inspiration from `IntConverter`, or even find an efficient way to share code between the two!
  
  Add the new converter to the list of [Built-in Converters](https://falcon.readthedocs.io/en/stable/api/routing.html#built-in-converters) under the `float` identifier.
  
  Open questions: should we support converting `nan`, `inf` & `-inf` from path?