Propagate all errors via Result and use vim.notify in lua for logging

[dasupradyumna]

Context

All notifications published by the plugin are handled using the internal utils::notify module. This module dispatches the notification to the builtin neovim API nvim_notify() function, which works as expected. But in the future, the plugin should support notification decorator plugins such as nvim-notify, which overload the builtin vim.notify() function. This behavior will not be supported by the current notification module.

Proposal

Implement a separate notification module in lua, using vim.notify() function. This would require the API functions return a notification message along with the level. This can be achieved by propagating all errors using Result objects internally, and handling them at the top-level.

[dasupradyumna]

Blocked by #18.