performable: prefix (#4345)

closes #4328
closes #3970

makes this possible now
```
keybind = performable:ctrl+c=copy_to_clipboard # copy if theres a selection else send sigint
keybind = ctrl+v=paste_from_clipboard
```

src/input/Binding.zig

@@ -36,6 +36,11 @@ pub const Flags = packed struct {
     /// and not just while Ghostty is focused. This may not work on all platforms.
     /// See the keybind config documentation for more information.
     global: bool = false,
+
+    /// True if this binding should only be triggered if the action can be
+    /// performed. If the action can't be performed then the binding acts as
+    /// if it doesn't exist.
+    performable: bool = false,
 };
 
 /// Full binding parser. The binding parser is implemented as an iterator
@@ -90,6 +95,9 @@ pub const Parser = struct {
             } else if (std.mem.eql(u8, prefix, "unconsumed")) {
                 if (!flags.consumed) return Error.InvalidFormat;
                 flags.consumed = false;
+            } else if (std.mem.eql(u8, prefix, "performable")) {
+                if (flags.performable) return Error.InvalidFormat;
+                flags.performable = true;
             } else {
                 // If we don't recognize the prefix then we're done.
                 // There are trigger-specific prefixes like "physical:" so
@@ -1688,6 +1696,16 @@ test "parse: triggers" {
         .flags = .{ .consumed = false },
     }, try parseSingle("unconsumed:physical:a+shift=ignore"));
 
+    // performable keys
+    try testing.expectEqual(Binding{
+        .trigger = .{
+            .mods = .{ .shift = true },
+            .key = .{ .translated = .a },
+        },
+        .action = .{ .ignore = {} },
+        .flags = .{ .performable = true },
+    }, try parseSingle("performable:shift+a=ignore"));
+
     // invalid key
     try testing.expectError(Error.InvalidFormat, parseSingle("foo=ignore"));