Most Linux environments already have this, it's called "compose key". You may need to enable it in your desktop environment's settings, but then after pressing the key you can press !? to type ‽, 12 to type ½, <= to type ≤, n~ to type ñ, <3 to type ♥, etc. Here's GTK's list: https://help.ubuntu.com/community/GtkComposeTable