Readd core examples

examples/core/input_keys.zig

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+//
+// input_keys
+// Zig version: 
+// Author: Nikolas Wipper
+// Date: 2020-02-16
+//
+
+const rl = @import("raylib");
+
+pub fn main() anyerror!void {
+    // Initialization
+    //--------------------------------------------------------------------------------------
+    const screenWidth = 800;
+    const screenHeight = 450;
+
+    rl.InitWindow(screenWidth, screenHeight, "raylib-zig [core] example - keyboard input");
+
+    var ballPosition = rl.Vector2 { .x = screenWidth/2, .y = screenHeight/2 };
+
+    rl.SetTargetFPS(60);               // Set our game to run at 60 frames-per-second
+    //--------------------------------------------------------------------------------------
+
+    // Main game loop
+    while (!rl.WindowShouldClose())   // Detect window close button or ESC key
+    {
+        // Update
+        //----------------------------------------------------------------------------------
+        if (rl.IsKeyDown(rl.KeyboardKey.KEY_RIGHT)) { ballPosition.x += 2.0; }
+        if (rl.IsKeyDown(rl.KeyboardKey.KEY_LEFT)) { ballPosition.x -= 2.0; }
+        if (rl.IsKeyDown(rl.KeyboardKey.KEY_UP)) { ballPosition.y -= 2.0; }
+        if (rl.IsKeyDown(rl.KeyboardKey.KEY_DOWN)) { ballPosition.y += 2.0; }
+        //----------------------------------------------------------------------------------
+
+        // Draw
+        //----------------------------------------------------------------------------------
+        rl.BeginDrawing();
+
+            rl.ClearBackground(rl.RAYWHITE);
+
+            rl.DrawText("move the ball with arrow keys", 10, 10, 20, rl.DARKGRAY);
+
+            rl.DrawCircleV(ballPosition, 50, rl.MAROON);
+
+        rl.EndDrawing();
+        //----------------------------------------------------------------------------------
+    }
+
+    // De-Initialization
+    //--------------------------------------------------------------------------------------
+    rl.CloseWindow();        // Close window and OpenGL context
+    //--------------------------------------------------------------------------------------
+}