terminal: respond to XTVERSION query

XTVERSION (CSI > 0 q) is used by some libraries to identify the terminal
+ version. Respond to this query with `ghostty {version_string}`. There
is no formal format for this response. A roundup of a few tested
terminals show two primary formats. This patch opts to save one byte and
use the `name SP version` semantics.

foot: foot(version)
xterm: XTerm(version)
contour: contour version
wezterm: wezterm version

Reference: https://github.com/dankamongmen/notcurses/blob/master/TERMINALS.md#notes-for-terminal-authors
Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>

src/termio/Exec.zig

@@ -1688,6 +1688,23 @@ const StreamHandler = struct {
         self.terminal.screen.kitty_keyboard.set(mode, flags);
     }
 
+    pub fn reportXtversion(
+        self: *StreamHandler,
+    ) !void {
+        log.debug("reporting XTVERSION: ghostty {s}", .{build_config.version_string});
+        var msg: termio.Message = .{ .write_small = .{} };
+        const resp = try std.fmt.bufPrint(
+            &msg.write_small.data,
+            "\x1BP>|{s} {s}\x07",
+            .{
+                "ghostty",
+                build_config.version_string,
+            },
+        );
+        msg.write_small.len = @intCast(resp.len);
+        self.messageWriter(msg);
+    }
+
     //-------------------------------------------------------------------------
     // OSC