Icon highlight separation

[tostrstrudl]

feat: allow separate highlight for completion item icon (#1599)

This PR adds support for separately highlighting the completion item icon (kind)
without affecting the abbreviation (abbr), as requested in #1599.

• Added another field ('icon') to allow for separate icon movement and highlighting
• Added new highlight groups for the icons (kind specific, icon field specific, and default)
• Added one require() line for LspKind and a 4-line function to pull a kind's icon (get_icon()); they default to cmp_opts.icon or ' ' if LspKind is not present or there are no custom-defined icons
• Updated cmp.txt to inform of the new highlight fields with an example on how to set them
• Tested with my own local configuration of nvim-cmp/lspkind to be sure that everything loads and runs as it should (no breaking changes)

Closes #1599.