Put the sensible-terminal script into i38 itself.

There is no such --sensible-terminal flag to Sway, and given it's just
a script we may as well put it in our codebase instead.

scripts/i3-sensible-terminal

@@ -0,0 +1,23 @@
+#!/bin/sh
+#
+# This code is released in public domain by Han Boetes <han@mijncomputer.nl>
+#
+# This script tries to exec a terminal emulator by trying some known terminal
+# emulators.
+#
+# We welcome patches that add distribution-specific mechanisms to find the
+# preferred terminal emulator. On Debian, there is the x-terminal-emulator
+# symlink for example.
+#
+# Invariants:
+# 1. $TERMINAL must come first
+# 2. Distribution-specific mechanisms come next, e.g. x-terminal-emulator
+# 3. The terminal emulator with best accessibility comes first.
+# 4. No order is guaranteed/desired for the remaining terminal emulators.
+for terminal in "$TERMINAL" x-terminal-emulator mate-terminal gnome-terminal terminator xfce4-terminal urxvt rxvt termit Eterm aterm uxterm xterm roxterm termite lxterminal terminology st qterminal lilyterm tilix terminix konsole kitty guake tilda alacritty hyper wezterm; do
+    if command -v "$terminal" > /dev/null 2>&1; then
+        exec "$terminal" "$@"
+    fi
+done
+
+i3-nagbar -m 'i3-sensible-terminal could not find a terminal emulator. Please install one.'