feat: add file icons for .lean files (#749)

Author: mhuisi

vscode-lean4/media/lean-file-icon-dark.png

@@ -789,7 +789,11 @@
                 "configuration": "./language-configuration.json",
                 "extensions": [
                     ".lean"
-                ]
+                ],
+                "icon": {
+                    "dark": "./media/lean-file-icon-dark.png",
+                    "light": "./media/lean-file-icon-light.png"
+                }
             },
             {
                 "id": "lean",