Fix syntax highlighting for matchers using parentheses

[asymmetric]

Hi,

As discussed in this PR, the correct syntax for Gherkin matchers is

Given(/This and that/)

And not

Given /This and that/

because the latter is ambiguous and causes ruby to issue the warning ambiguous first argument; put parentheses or even spaces. (note: funny that the GitHub syntax highlight makes the same mistake)

It would be great if the syntax file in this plugin could highlight the correct case (and possibly, stop highlighting the wrong one!)

Thanks for all your great plugins ❤️

[Tim Pope]

These runtime files are concerned with .feature files only.