Clear chat prompt logic has been moved to dedicated method in Chat class to
improve code reusability and maintainability. Window local cwd detection
has been simplified and now uses VimEnter/WinEnter events instead of
DirChanged for more reliable current working directory tracking.

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

Turns out nvim_win_get_var was a drama queen, throwing exceptions
whenever it couldn't find its precious variable - like a toddler having
a meltdown when they can't find their favorite toy.

Switched to vim.w, the more chill cousin who just returns nil and goes
about their day when something's missing. Now our code won't have an
existential crisis just because cchat_cwd wasn't set for one window.

No more exception-al behavior, just peaceful, quiet coding.