Replaces custom VM with custom "Urbit" element

[elliotBraem]

This is a continuation of #25.

To improve the developer experience and align the other "custom VM" implementations (Livepeer, Vara Network), this pull request replaces the near-social-vm with the most recent version and exposes a <Urbit /> custom element (see Configuring custom VM elements).

This custom element is a higher order component that has a "provides" prop, in order to pass helpful Urbit methods from the gateway to any widgets visible through the gateway. For example, for Poke:

<Urbit
  provides={({ poke }) => (
    <button
      onClick={() =>
        poke("hood", "helm-hi", "hello urbit!").then((res) => {
          console.log(`${pokeVal} been printed in dojo`);
        })
      }
    >
      poke
    </button>
  )}
/>

Although it may be clearer when used like a "Provider" to a Widget, like this:

<Urbit
    provides={({ poke }) => (
      <Widget
        src={"${config_account}/widget/components.pokeUrbit"}
        props={{ poke }}
      />
    )}
  />

Then poke is accessible via props.

This pattern removes the need for a custom VM (and no longer needs to update and adjust for security vulnerabilities), and is more convenient for initializing the api as this can be done by the gateway, rather than inside the widget. It removes the "setApi" method.

It also provides developers with an easier to manage + customize Urbit API within the gateway code itself, rather than needing to modify the VM