`data_dir` saves the absolute path

[gcstr]

I sync some folders between machines and I'd like to be able to do that with mind files as well.
The issue is that mind expands the persistence location to an absolute path in the json file.

My config has

  require('mind').setup({
    persistence = {
      state_path = vim.fn.expand('~') .. "/Sync/mind.json",
      data_dir = vim.fn.expand('~') .. "/Sync/mind",
    },
  })

resulting to

{
	"tree": {
		"is_expanded": true,
		"children": [{
			"is_expanded": true,
			"children": [{
				"is_expanded": true,
				"children": [{
					"is_expanded": true,
					"data": "\/home\/gcstr\/Sync\/mind\/20221226232901-notes.md",
					"contents": [{
						"text": "Notes"
					}]
				}],
...

This renders my mind files unusable on Mac.

Maybe we could move the expansion to the line when the arg is read:
https://github.com/phaazon/mind.nvim/blob/42726da19eac75ac373aca4424dcc5536a14a54b/lua/mind/init.lua#L91

Instead of writing the expanded path here:

https://github.com/phaazon/mind.nvim/blob/a8de4150143cba275bccdd6a184ebb5650a5f377/lua/mind/state.lua#L12