Add alt keybindings for ctrl+ins = Copy and shift+ins = Paste (#2870)

On non-MacOS desktop environments (Windows, Gnome, KDE, Xfce, VS Code,
...), `ctrl+ins` and `shift+ins` are system-wide alternate keybindings
(except for terminals) for `Copy` and `Paste` respectively. This PR
explicitly defines them as such in Ghostty's default keybindings.

Using `ctrl+ins` as an alt-keybinding for `Copy` allows
static/context-unaware remapping of `Copy` to `cmd+c` for Linux systems
using Mac keyboards via
[keyd](NixOS/nixpkgs#137698 (comment));
with the default `ctrl+shift+c` keybinding for copy this is basically
impossible (because that binding only applies to terminals).

src/config/Config.zig

@@ -2083,6 +2083,20 @@ pub fn default(alloc_gpa: Allocator) Allocator.Error!Config {
             .{ .key = .{ .translated = .v }, .mods = mods },
             .{ .paste_from_clipboard = {} },
         );
+        // On non-MacOS desktop envs (Windows, KDE, Gnome, Xfce), ctrl+insert is an
+        // alt keybinding for Copy and shift+ins is an alt keybinding for Paste
+        if (!builtin.target.isDarwin()) {
+            try result.keybind.set.put(
+                alloc,
+                .{ .key = .{ .translated = .insert }, .mods = .{ .ctrl = true } },
+                .{ .copy_to_clipboard = {} },
+            );
+            try result.keybind.set.put(
+                alloc,
+                .{ .key = .{ .translated = .insert }, .mods = .{ .shift = true } },
+                .{ .paste_from_clipboard = {} },
+            );
+        }
     }
 
     // Increase font size mapping for keyboards with dedicated plus keys (like german)