[Godot 4] Fix editor terminal shortcuts

[lihop]

Default shortcuts:

• Copy: Ctrl+Shift+C
• Paste: Ctrl+Shift+P
• Kill focused terminal: Ctrl+Shift+X
• Open new terminal: Ctrl+Shift+T
• Tab left: Ctrl+Shift+PgUp
• Tab right: Ctrl+Shift+PgDown

All of these use the Shift modifier as to not conflict with control character input. For example, Ctrl+C is used to write the "end of text" control character.

These shortcuts should also be shown (in brackets) in the "right-click" popup menu:

As a bonus, these shortcuts should be configurable in editor settings (see #64). Shortcut settings are not currently exposed by the engine, but it looks like there are plans to expose them in the future. In the meantime, we could use string representations of the shortcuts and parse them using OS.find_keycode_from_string().