patch 9.1.1138: cmdline completion for :hi is too simplistic

patch 9.1.1138: cmdline completion for :hi is too simplistic

Problem:  Existing cmdline completion for :highlight was barebone and
          only completed the highlight group names.

Solution: Implement full completion for the highlight group arguments
          such as guifg and cterm. If the user tries to complete
          immediately after the '=' (e.g. `hi Normal guifg=<Tab>`), the
          completion will fill in the existing value, similar to how
          cmdline completion for options work (Yee Cheng Chin).

closes: #16712

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>