Raise error if there are multiple matches

Similar to AmbiguousMatchError in Capybara

If you do `click "@something"` and there are 2 or more elements that match, raise `LuckyFlow::AmbiguousMatchError`

The error message should give pointers on what's going on and how to resolve it. For example:

```
There were X matches for "#{the_selector}" so LuckyFlow isn't sure which one you meant.

Try this...

* Use a more specific selector: ".my-wrapper-element #{the_selector}"
```

Also, add a `click_first` method to just click the first one

Start at the click entry point and inspect how matching and existing errors are handled, using Capybara's AmbiguousMatchError as the stated reference. Add coverage for multiple matches, the suggested error guidance, and click_first; done when ambiguous clicks raise the named error and first-match clicks are supported.

Written by the indexing model from the issue text.