Problem:
For a given position, it is not easy to compare which of several other positions is closest to it.
Solution:
Add support for converting `vim.Pos` to a buffer byte offset.
This allows for sorting, e.g:
```lua
table.sort(positions, function(pos1, pos2)
return pos1:to_offset() < pos2:to_offset()
end
```
Or a binary search, e.g:
```lua
vim.list.bisect(positions, pos, { key = function(pos) return pos:to_offset() end })
```
Co-authored-by: Yi Ming <ofseed@foxmail.com>
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