fix(output_panel): recreate window and term channel if panel closed manually

[YaroSpace]

A fix for 2 issues that appear if output_panel is closed by the user manually and not with output_pannel.toggle/close():

1. The buffer id for output panel becomes invalid, so there is an error when trying to open the panel again. Currently the check with vim.fn.bufexists() does not catch buffers that exist, but are unloaded. Fix - recreate the buffer.
2. The channel id for term bound to panel buffer becomes invalid, so there is an error when trying to write to that channel. Fix - recreate the term.

Also, a fix with associated issue: if you reaload the plugin with say Lazy reload neotest, the old panel buffer still exists, so when a new buffer is created with the same name, there is an error of trying to rename existing buffer. Fix - delete buffer with the same name, when ceating a new one.