[nre] `captures[i]` should return `Option[string]` instead of recent subtly-breaking change that throws

[flaviut]

From nim-lang/RFCs#109 (comment)

/cc @flaviut
after this recent PR nim-lang/Nim#9755 some of my code broke, but it broke at RT instead of CT, which is not as good as if it failed at CT (because no-one has 100% test coverage, and who knows what other projects would break in the wild...)

how about the following:

* `m.captures[0]` returns the standard `Option[string]` instead of this non-standard `[]` that returns a `string` but throws on un-captured group
  that would've forced an nre user to fix his code and guaranteed it would work 100% ; right now there is no such warrantee until client code hits such an issue in the wild

* furthermore, `Option[string]` leads to cleaner code, eg:

m.captures[0].get("") # case 1: uses options.get with default of `""` for cases where user doesn't care if it's empty vs non-captured, ie matches behavior prior to this PR
m.captures[0].get # case 2: will throw in case of non-captured

where with code after nim-lang/Nim#9755 it's less clean:

if 0 in m.captures: m.captures[0] else: "" # case 1
m.captures[0] # case 2
# note, case 1 could use a helper proc (say, `getOrDefault`) but it'd still not be as clean as if we used `Option`:
m.captures.getOrDefault(0,"")

basically, with Option, things are more composable (and can reuse functionality in std/options); with current API, it's written more awkwardly.

Alternative (for a smoother transition via deprecation path compared to this PR)

keep the old captures (that returns "" instead of nil for no capture) but mark it as deprecated, and add a new capturesOption (or whatever name) that returns an Option[string]

Note

Option[string] would just make everything more consistent, eg with captures.toSeqand captures.toTable that must return seq[Option[string]] anyway /cc @dom96