Fix end-of-block comments not highlighting

Originally, comments in code blocks were highlighted by looking for
their starting token at the beginning and a newline at the end.

This PR adds two new comment patterns that look for the end-of-string
instead of a newline, so that comments on the last line of a block are
still highlighted.

ldoc/lexer.lua

@@ -305,6 +305,7 @@ function lexer.lua(s,filter,options)
             {'^`[^`]+`',bdump},
             {'^%-%-%[(=*)%[.-%]%1%]',cdump},
             {'^%-%-.-\n',cdump},
+            {'^%-%-.-$',cdump},
             {'^%[(=*)%[.-%]%1%]',sdump_l},
             {'^==',tdump},
             {'^~=',tdump},
@@ -354,6 +355,7 @@ function lexer.cpp(s,filter,options,no_string)
             {STRING1,chdump},
             {STRING2,sdump},
             {'^//.-\n',cdump},
+            {'^//.-$',cdump},
             {'^/%*.-%*/',cdump},
             {'^==',tdump},
             {'^!=',tdump},