feat: Implement long-form switch parsing

This adds support for long-form switches; a long-form switch works similarly
to a flag switch, but the switch itself is identified by a string encompassing
multiple characters.

To enable a long-form switch, pass its name preceded by a double dash, like so:

```bash
$ node program --long-form-switch
```

Since long-form switches can have arbitrarily long names, they can't be grouped
like flag switches are, and each switch requires a new argument:

```bash
$ node program --switch-one --switch-two
```