fix function signatures, to ensure compilation without warnings with latest V

[spytheman]

With this PR:

#0 19:33:08 ᛋ master ~/.vmodules/srackham/pcre2❱v run examples/pcre2-example-1.v 
['nisi', 'dis', 'diam', 'natoque']
Lorem "nisi" "dis" "diam" a cras placerat "natoque"
Lorem NISI DIS DIAM a cras placerat NATOQUE
Lorem NISI DIS DIAM a cras placerat NATOQUE
#0 19:33:26 ᛋ master ~/.vmodules/srackham/pcre2❱

i.e. there are no more warnings.

[srackham]

Thanks for the PR and for alerting me to this issue.

The PR changes the find_match function signature so that instead of returning none on no match (PCRE2_ERROR_NOMATCH) it returns an error with a blank string error message. This fixes the error warning but it's kind of weird to class no match as an error.

I've pushed a commit (98f2fbe) that retains the current behaviour (returns none when there is no-match). The return of none is in keeping with Go regexp package convention of returning nil on no match.

I also updated the internal get function to conform to the new V split Option/Result behaviour.