fix: Use winborder for window menu and fix scrollbar window

[Aqothy]

Use win border as the default border option in config, also fix scrollbar window

scrollbar before:

scrollbar after (also with rounded win border):

[Aqothy]

my first open source contribution, would love if this could be merged :) issue: #2159

[wsdjeg]

same as #2150

[Aqothy]

same as #2150

hey this pr also adds the feature to use the win border option as the default border option in cmp config, so if user has vim.opt.winborder = "rounded" they dont have to configure nvim cmp to use rounded borders using cmp.config.window.bordered(), up to you if you want this to be the default though

[wsdjeg]

Thanks, I just test it, and it works well for me, merged.