refactor!: use OO & add tests

[delphinus]

I did an overhall for all codes and am refactoring them with OO-like format and typing by Lua-language-server. I will add unit & integration tests for robustness.

You can try the new version of frecency with settings below. The old version can use with this.
I have deleted the old logic in a15766e. You can simply switch this branch to use the new logic.

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TODO

• Realize the same behavior with the old version
• Treat .gitignore in scandir
• Treat auto_validate
• Treat disable_devicons
• Health check
• There is a bug as below.
  1. Fetch results when with :Telescope frecency
  2. Then do :Telescope frecnecy workspace=CWD, it does not filter the results with workspace.
• A bug as below.
  1. :Telescope frecency
  2. Input :CWD: to filter out.
  3. Delete :CWD: and it should show the first look again, but it does not.
• Uas async finder not to block Neovim in searching workspace (Fix Calling plenary.scandir is slow #106)
  • Use vim.fs.dir instead of plenary.scandir.
  • Completely async finder. See async_finder.lua.
• Make count = 1 when you open a file you've never opened (Fix A file opened at the first time is recorded count = 0 #107)
• More tests
  • recency
  • finder
    • ignore_patterns
  • frecency
• Remove duplications between entries from the database and ones from the workspace.

less priority TODO

• Load dependencies lazily → Load dependencies lazily #109
  • This is needed for loading telescope-frecency.nvim itself lazily. We need frecency:register() to load at first, but does not need dependencies until :Telescope frecency starts.
• Cache each entry instead of the whole results. → Cache each entry instead of the whole results. #110
  • The current build shows the same score after the entry is opened.
• Ignore cases in completing tags. → Ignore cases in completing tags #111