feat: add logic to store data by native code

[delphinus]

ref #91

How to use

1. Change setup opts

telescope.setup {
  extensions = {
    frecency = {
      use_sqlite = false,  -- default value is `true`.
    },
  },
}

2. Run migration command

:FrecencyMigrateDB

Then it uses DB without SQLite. You can drop sqlite.lua.

TODO

• [x] A way to select logic between SQLite and string.dump().
• [x] Lock and unlock file in accessing DB.
  • This is needed because multi Neovim instances can access DB at the same time. We need a background thread for queuing jobs to save data into DB.
• [x] Migration script.
  • need auto converting?
• [x] Load sqlite lazily not to require it in bootstrap.
• [x] Remove healthcheck for sqlite.lua
• [x] In load() or save(), it should check the file is not locked and should wait for unlocking when it is locked.
• [x] Note in README.

Issues I solves later

• Call save() asynchronously for performance
• Refresh data in memory when DB has been updated.
  • It can detect DB to be updated by the file's mtime.
  • Poll the file by uv.new_fs_event is better than above.