Fix repl_splitter for first char in newline being non-ascii

[sebastianpech]

Fixes StringIndexError in consolelexer when the first character after a linebreak is a non-ascii character.

In the original code prevind(s, i - 1), i-1 had no effect for non-ascii characters because i-1 gives an invalid index that is still within the character.

julia> prevind("abβc",5)
3

julia> prevind("abβc",5-1)
3

So to go back 2 indices this must be replaced with prevind(s, i, 2).

[KristofferC]

Would be good with a test (added to https://github.com/JuliaDocs/Highlights.jl/blob/master/test/lexers/jlcon.jl).

[mortenpi]

LGTM, thanks!