feat: add functions for darkening/lightening highlight groups

feat: add functions for darkening/lightening highlight groups

I'm not documenting these yet as I they are probably riddled with bugs,
but they seem to work in the simple test cases that I have tried out so
far. Specifically, I'm doing this in my dotfiles to make floating
windows look better:

    local dark = vim.o.background == 'dark'
    local factor = dark and 0.15 or -0.15
    local normal = pinnacle.adjust_lightness('Normal', factor)
    vim.cmd('highlight! clear NormalFloat')
    vim.cmd('highlight! NormalFloat ' .. pinnacle.highlight(normal))
    normal['fg'] = dark and '#ffffff' or '#000000'
    vim.cmd('highlight! clear FloatBorder')
    vim.cmd('highlight! FloatBorder ' .. pinnacle.highlight(normal))