Use "state" pattern in `suite.runSteps`

The [`runStep` function](https://github.com/cucumber/godog/blob/b850b44b48b7cb1ff1668b2c789084251f6fd81a/suite.go#L75) has a bunch of conditional logic in it, which I think could be simplified by the application of the [State](https://sourcemaking.com/design_patterns/state) design pattern.

We use this [in the Ruby version](https://github.com/cucumber/cucumber-ruby-core/blob/main/lib/cucumber/core/test/runner.rb#L96), and pretty much eliminated every `if` statement, meaning a whole class of bugs like https://github.com/cucumber/godog/issues/434 disappeared.

What do folks think? Could we do something like this? Would it be useful?

Start by reading suite.go, especially the runStep function linked in the issue, and compare it with the Ruby runner implementation. Trace the conditional states and review issue #434 for the bug context. Done means agreeing on a useful State-pattern design and implementing it without changing runSteps behavior, with tests covering the affected transitions.

Written by the indexing model from the issue text.