Fix accidental dedent for in and when dedent in Ruby comments

[vinistock]

I was testing the changes from #205397 in Insiders and I noticed that the code was incorrectly being dedented when typing in or when in comments.

We got the parenthesis balance incorrect in the regex. The regex has three outer groups for an or condition and we closed the group incorrectly, which made the last part of the regex skip the \s* part (which verifies that only blank spaces are in the front of the string).

This PR fixes the bug by properly balancing the parenthesis. The tests was also missing the ^ anchor, which I added to match the language configuration.

cc @rebornix

[rebornix]

@vinistock thanks for the quick fix! Does it only affect comments? We are in endgame mode so I'm trying to understand if it's worth a candidate or if we should pull back the previous change and merge together with this one.

[vinistock]

@rebornix because of the unbalanced parenthesis, it ignores the \s* part, meaning it matches any in|when words - not just comments. Even if you start writing a method call like within and put a space, it will incorrectly dedent.

I don't know what the endgame policies are, but in my opinion it's definitely worth either pulling back the previous change or including this one.