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...

https://github.com/MarcHamamji/runner.nvim/blob/2c1929f02f199e5744f96ef04a1bb412d859e16c/lua/runner/handlers/utils.lua#L23