md-tools.nvim/lua/md-tools/init.lua

local M = {}
local header_regex = "^(#+) (.+)"

M.promote = function()
	local current_line = vim.api.nvim_get_current_line()
	local row = vim.api.nvim_win_get_cursor(0)[1]

	if current_line:match(header_regex) then
		if current_line:match("^# (.+)") then
			vim.notify("You can't promote this header anymore")
		else
            vim.api.nvim_buf_set_text(0, row - 1, 0, row - 1, 1, { '' })
		end
	end
end

M.demote = function()
	local current_line = vim.api.nvim_get_current_line()
	local row = vim.api.nvim_win_get_cursor(0)[1]

	if current_line:match(header_regex) then
		if current_line:match("^###### (.+)") then
			vim.notify("You can't demote this header anymore")
		else
            vim.api.nvim_buf_set_text(0, row - 1, 0, row - 1, 0, { '#' })
		end
	end
end

return M