feat: max_height for completion window

[dvimaaniye]

• Max height can be set separately from 'pumheight'
• Default behaviour is still to use pumheight, so existing configs won't be affected.
• A negative max_height will not display the menu. (Should it fallback to use all available space?)

	cmp.setup({
		...
		window = {
			completion = cmp.config.window.bordered({ max_height = 5 }), -- ignores pumheight
			-- completion = cmp.config.window.bordered({ max_height = 0 }), -- Uses all available space, ignores pumheight 
			-- completion = cmp.config.window.bordered(), -- Uses pumheight, default
			-- completion = cmp.config.window.bordered({ max_height = nil }), -- Uses pumheight, same as default
			
			-- completion = cmp.config.window.bordered({ max_height = -5 }), -- Menu not displayed
		},
		...
	})

Screenshots:
(max_height set to 5):

(max_height set to 0):

(max_height set to 5, pumheight set to 1, ignores pumheight):

(max_height set to nil (default), pumheight set to 1, uses pumheight):

[wsdjeg]

Thanks, LGTM, merged.