Highlight backtrace level

[albfan]

Although up and down trough stack level is not yet possible

http://vimdoc.sourceforge.net/htmldoc/repeat.html#debug-scripts

NOTE: The debugging mode is far from perfect. Debugging will have side effects on how Vim works. You cannot use it to debug everything. For example, the display is messed up by the debugging messages. {Vi does not have a debug mode}

Highlight it for future implementations

[albfan]

albfan/vim#2 provides support to "up" and"down" debug commands