refactor(date): improve type hints

[seflue]

• introduce types for internal date tables
• introduce some helper to encapsulate interaction with os api

[seflue]

@kristijanhusak As promised here the cleaned up type hints in Date.
The failing test is unrelated, but when I had a look at it, it seems to be a flaky candidate. I don't think it can be made 100% deterministic, but at least if we cache the current time right after the command (before vim.wait we can reduce the probability to fail:

orgmode/tests/plenary/ui/mappings/todo_spec.lua

Lines 343 to 357 in ac9a6e0

	vim.fn.cursor(3, 3)
	vim.cmd([[norm citd]])
	vim.wait(50)
	assert.are.same({
	'* PHONECALL Daily stand-up with the team',
	' SCHEDULED: <2021-09-08 Wed 09:00 +1d>',
	' :PROPERTIES:',
	' :REPEAT_TO_STATE: PHONECALL',
	' :LAST_REPEAT: [' .. Date.now():to_string() .. ']',
	' :END:',
	' :LOGBOOK:',
	' - State "DONE" from "MEET" [' .. Date.now():to_string() .. ']',
	' :END:',
	}, vim.api.nvim_buf_get_lines(0, 2, 11, false))

[kristijanhusak]

Generally looks ok. I just skimmed through it. See if we need to address some of these comments and then I'll check it thoroughly.

[kristijanhusak]

Ok this looks good now. I'll do some minor changes after merging, but I don't want to bother you with those nits. Thanks!