add some treesitter color overrides

The default colors for some of these things are unreadable when the
theme is in "light" mode. For others, I think the new choice is a bit
better, e.g. making function calls bold.

Minor refactoring

Sorts some things, and changes definitions of some other things to be
more generic. Should have no functional changes

improve TODO highlight colors

color Todo

simplify color definitions, drop 'light' theme, better highlight for ()

add color for IndentBlanklineChar

add colors for GitGutter{Add,Change,Delete}

use a better yellow for selection

Set completion menu selection colors

initial commit