Colour for column selection in debugging mode added to high contrast …

fixes #20622

fixes #20601

fixes #20603

Part of #18454

Fixes #19026

peekLF and peekSingleByteChar apparently aren't inlined, hurting the perf in this very hot loop. The perf regression is partly from function calls, and partly from the function depending on a closure. It also seems like there might be more gains to be had if I refactor this file to not depend so heavily on closures and nested anonymous functions.

* markdown-it-named-header custom slugify for non-latin characters

* Delete comment

Terminal links

fixes #18672