Investigate incomplete method call behaviour

[amomchilov]

class Before
end

class Current
  x.
end

class After; end

Desired:

class Before
end

class Current
  x.<method-name-missing>
end

class After; end

Prism treats like:

class Before
end

class Current
  x.end
  class After; end
end

[amomchilov]

Did some pairing with @Morriar on this. Looks like it's two related issues:

1. How Prism recovers from a missing end.
2. How an end could be misinterpreted as method name, causing a missing end error.

It looks like we'll need to introduce some notion of backtracking to Prism to solve this correctly. For comparison, here's the PR that added "rewind" t othe Sorbet parser sorbet#5214

And here's where that message is diagnosed:

https://github.com/Shopify/prism/blob/69abcdbb18f34908dd379429353c341f9477aef7/src/prism.c#L20846-L20849