WIP: Support Persist Bookmarks

[xiantang]

close #1851 (comment)

[alex-courtis]

This looks great! I'll give it a full review and test in the next few days.

Initial thoughts:

• Should we be using vim.json.encode instead of the fn ?
• Please don't move existing methods (constructor, toggle)
• This should be an optional feature in nvim-tree-opts, disabled by default
• This needs checking and safety
  • Use a pcall or similar to prevent an error being raised
  • Report failures to the user