refactor(offsets): use wininfo to calculate offset

[akinsho]

fixes #789

[nullptr-yxw]

It still cannot work as expect when the offset is the only window

2023-08-05.00-55-22.mp4

my configuration of options.offsets is

offsets = {
    {
        filetype = "NvimTree",
        text = "File Explorer",
        highlight = "Directory",
        text_align = "left",
        separator = true,
    },
},