Reuse shell_hander window

[MarcHamamji]

This code should reuse the previous shell-handler window if it's still open, but I don't know why vim.api.nvim_win_is_valid(M._terminal_window) is always false...

runner.nvim/lua/runner/handlers/utils.lua

Line 23 in 2c1929f

if M._terminal_window and vim.api.nvim_win_is_valid(M._terminal_window) then