Implement CLI --export-svg parameter for instant SVG export

Co-authored-by: enaut <290005+enaut@users.noreply.github.com>

turtle-lib/examples/test_svg_export.rs

@@ -0,0 +1,36 @@
+//! Test example for CLI SVG export feature
+//!
+//! Run this with: cargo run --package turtle-lib --example test_svg_export --features svg -- --export-svg test_output.svg
+
+use turtle_lib::*;
+
+#[turtle_main("SVG Export Test")]
+fn draw_test(turtle: &mut TurtlePlan) {
+    turtle.set_pen_color(RED);
+    turtle.set_pen_width(3.0);
+    
+    // Draw a square
+    for _ in 0..4 {
+        turtle.forward(100.0);
+        turtle.right(90.0);
+    }
+    
+    // Draw a circle
+    turtle.set_pen_color(BLUE);
+    turtle.pen_up();
+    turtle.forward(150.0);
+    turtle.pen_down();
+    turtle.circle_left(50.0, 360.0, 36);
+    
+    // Draw a filled triangle
+    turtle.set_fill_color(GREEN);
+    turtle.pen_up();
+    turtle.go_to(vec2(-50.0, 100.0));
+    turtle.pen_down();
+    turtle.begin_fill();
+    for _ in 0..3 {
+        turtle.forward(80.0);
+        turtle.right(120.0);
+    }
+    turtle.end_fill();
+}