[ruby/syntax_suggest] Clean up output

I previously left a comment stating I didn't know why a certain method existed. In investigating the code in `CaptureCodeContext#capture_before_after_kws` I found that it was added as to give a slightly less noisy output.

The docs for AroundBlockScan#capture_neighbor_context only describe keywords as being a primary concern. I modified that code to only include lines that are keywords or ends. This reduces the output noise even more.

This allows me to remove that `start_at_next_line` method.

One weird side effect of the prior logic is it would cause this code to produce this output:

```
        class OH
          def hello

          def hai
          end
        end
```

```
          1  class OH
        > 2    def hello
          4    def hai
          5    end
          6  end
```

But this code to produce this output:

```
        class OH
          def hello
          def hai
          end
        end
```

```
          1  class OH
        > 2    def hello
          4    end
          5  end
```
Note the missing `def hai`. The only difference between them is that space.

With this change, they're now both consistent.

https://github.com/ruby/syntax_suggest/commit/4a54767a3e

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