[example] Add shaders_rounded_rectangle example (#4719)

* Add shaders_rounded_rectangle example

* Minor tweaks to example template (add star, more)

* Combine shaders

* Fix changes after review

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Co-authored-by: Anstro Pleuton <anstropleuton@github.com>

examples/Makefile.Web

@@ -514,6 +514,7 @@ SHADERS = \
     shaders/shaders_palette_switch \
     shaders/shaders_postprocessing \
     shaders/shaders_raymarching \
+    shaders/shaders_rounded_rectangle \
     shaders/shaders_shadowmap \
     shaders/shaders_shapes_textures \
     shaders/shaders_simple_mask \