Add achievement system with unlock and UI popup functionality (#8)

sprites/achievement.png.import

@@ -0,0 +1,34 @@
+[remap]
+
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+uid="uid://cebeyr4wnibvk"
+path="res://.godot/imported/achievement.png-6dc4e0e28c2cf29a2febc21fd42d98fb.ctex"
+metadata={
+"vram_texture": false
+}
+
+[deps]
+
+source_file="res://sprites/achievement.png"
+dest_files=["res://.godot/imported/achievement.png-6dc4e0e28c2cf29a2febc21fd42d98fb.ctex"]
+
+[params]
+
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+compress/high_quality=false
+compress/lossy_quality=0.7
+compress/hdr_compression=1
+compress/normal_map=0
+compress/channel_pack=0
+mipmaps/generate=false
+mipmaps/limit=-1
+roughness/mode=0
+roughness/src_normal=""
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