WHAT?

Destructured `customGeckoDriverPath` out of `params` in
`function start()`.

Additionally, updated `test.e2e.js` to test the three `webdriver` usage
modes: 1/ automatically download and start, 2/ automatically start, 3/
manually start.

Finally, added a self-referencing `devDependency` to `geckodriver`
since `webdriverio` transitively depends on it.

WHY?

Before this change, running the following code:

```ts
import { start } from 'geckodriver'

await start({ customGeckoDriverPath: '/some/path/geckodriver' })
```

Would fail to start the `geckodriver` because it would execute:

```shell
geckodriver --custom-gecko-driver-path=/some/path/geckodriver --port=43083 --host=0.0.0.0 --websocket-port=0
```

Which results in exit code `64` and the following output:

```log
/some/path/geckodriver: error: Found argument '--custom-gecko-driver-path' which wasn't expected, or isn't valid in this context

If you tried to supply `--custom-gecko-driver-path` as a value rather than a flag, use `-- --custom-gecko-driver-path`

USAGE:
    geckodriver [OPTIONS]

For more information try --help
```

By destructuring `customGeckoDriverPath`, the extraneous
`--custom-gecko-driver-path` argument is no longer specified and the
webdriver starts successfully.

HOW?

Successfully ran:

```shell
npm run build && npm test
```