* test: work tests for async_finder
I noticed these tests do not work because of vim.fs.joinpath lacking. I
will enable these tests and fix the bug.
* feat: implement vim.fs.joinpath to work in v0.9.0
* I did an overhall for all codes and added typing by Lua-language-server and tests. It also works on CI.
* Now it searches files on the workspace completely asynchronously. It does not block your text input. (Fix#106)
Make count = 1 when you open a file you've never opened (Fix#107)
