Allow optional OSC 52 parameter

Quoting from the XTerm documentation:

The first, Pc, may contain zero or more characters from the set c, p, q,
s, 0, 1, 2, 3, 4, 5, 6, and 7. It is used to construct a list of
selection parameters for clipboard, primary, secondary, select, or
cut-buffers 0 through 7 respectively, in the order given. If the
parameter is empty, xterm uses s 0 , to specify the configurable
primary/clipboard selection and cut-buffer 0.

See https://invisible-island.net/xterm/ctlseqs/ctlseqs.html

src/terminal/osc.zig

@@ -260,7 +260,12 @@ pub const Parser = struct {
             },
 
             .clipboard_kind => switch (c) {
-                ';' => self.state = .invalid,
+                ';' => {
+                    self.command.clipboard_contents.kind = 'c';
+                    self.state = .string;
+                    self.temp_state = .{ .str = &self.command.clipboard_contents.data };
+                    self.buf_start = self.buf_idx;
+                },
                 else => {
                     self.command.clipboard_contents.kind = c;
                     self.state = .clipboard_kind_end;
@@ -580,6 +585,20 @@ test "OSC: get/set clipboard" {
     try testing.expect(std.mem.eql(u8, "?", cmd.clipboard_contents.data));
 }
 
+test "OSC: get/set clipboard (optional parameter)" {
+    const testing = std.testing;
+
+    var p: Parser = .{};
+
+    const input = "52;;?";
+    for (input) |ch| p.next(ch);
+
+    const cmd = p.end().?;
+    try testing.expect(cmd == .clipboard_contents);
+    try testing.expect(cmd.clipboard_contents.kind == 'c');
+    try testing.expect(std.mem.eql(u8, "?", cmd.clipboard_contents.data));
+}
+
 test "OSC: report pwd" {
     const testing = std.testing;