fix assigning unpaired paren face to all unpared parens

[KristofferC]

Fixes #17

(underflow bad) :P.

julia> Base.annotations(JuliaSyntaxHighlighting.highlight("(())))"))
6-element Vector{@NamedTuple{region::UnitRange{Int64}, label::Symbol, value}}:
 @NamedTuple{region::UnitRange{Int64}, label::Symbol, value}((1:1, :face, :julia_rainbow_paren_1))
 @NamedTuple{region::UnitRange{Int64}, label::Symbol, value}((2:2, :face, :julia_rainbow_paren_2))
 @NamedTuple{region::UnitRange{Int64}, label::Symbol, value}((3:3, :face, :julia_rainbow_paren_2))
 @NamedTuple{region::UnitRange{Int64}, label::Symbol, value}((4:4, :face, :julia_rainbow_paren_1))
 @NamedTuple{region::UnitRange{Int64}, label::Symbol, value}((5:5, :face, :julia_unpaired_parentheses)) <----
 @NamedTuple{region::UnitRange{Int64}, label::Symbol, value}((6:6, :face, :julia_unpaired_parentheses)) <-----

[tecosaur]

Thanks for the catch + fix. I do wonder if this is the right approach, though? I chose UInt because negative values don't really have a useful meaning. Changing to Int stops the underflow, but also means that the number of unpaired closing parens changes the highlighting of all parens past that point.

I don't think this is the behaviour we want. Rather, I think we want to detect underflow and apply the unpaired paren face, but clamp the minimum value at 0 (in which case Int/UInt makes no difference).

[KristofferC]

Yeah, having a clamp makes sense (I added that + test). The issue with UInt is that if you accidentally underflow, you cannot detect the erroneous value (and it instead becomes a bogus valid value), and we don't care about the 2x extra space here.