Problem: vim._extui unconditionally creates windows, buffers and the
Vimscript cmdline highlighter when it is first loaded.
Solution: Schedule first creation of the window so that first redraw
happens sooner (still need to create at least the cmdline
window asap as it can have a different highlight through
hl-MsgArea; thus further delaying until the first event that
needs a particular target seems redundant). Load the cmdline
highlighter on the first cmdline_show event.