Signed-off-by: Seongmin Lee <git@boltless.me>

Using `guard()` function basically copied from fidget.nvim

Yeah `<...>` syntax doesn't work for GFM embedded videos
Who wouldn't know that.

- remove all global states
- provide more abstracted APIs to create new indicator
- generalize whole structure to include future UI components

- setup autocommands from `api.register()`
- some test tweaks
- rearrange some codes

more generalized way for indicators. Now user can make completely
different kind of indicator element other than statuscolumn and
virtualtext fro scratch.
fix: cut utf8 string in correct width with luautf8 luarocks module
fix: don't render when diagnostic is positioned on screen edge. It may
cover the existing diagnostic at that line.

- Include `BufEnter` action to handle `:tabnew` case
- Add proper logger
- More debug logs