Offer an option for type-stability of re_matches()

[MichaelChirico]

More than once now, we've run into an issue caused by type-instability of re_matches() output (logical vector vs. data.frame).

r-lib/lintr#196
r-lib/lintr#2287
r-lib/lintr#2678

At root, if we're writing functions taking regular expressions with re_matches() as a backend, it's hard to anticipate whether the user will use capture groups (or vice versa), and it can be hard to remember the need to branch the follow-up code depending on whether we received a data.frame.

Much better IMO to offer a path for type-stability directly in the re_matches() API (much like simplify=FALSE in *apply world). Maybe ignore_capture_groups (FALSE by default), then we can write code with ignore_capture_groups=TRUE to guarantee a logical return.