[rcore] [web] Adds SetWindowOpacity() implementation for PLATFORM_WEB

[ghost]

This implementation matches the desktop behavior: the whole canvas ("window") gets the opacity effect. It can be tested with:

#include "raylib.h"
int main(void) {
    InitWindow(800, 450, "test");
    SetTargetFPS(60);
    while (!WindowShouldClose()) {

        if (IsKeyPressed(KEY_ONE)) SetWindowOpacity(0.0f);
        if (IsKeyPressed(KEY_TWO)) SetWindowOpacity(0.5f);
        if (IsKeyPressed(KEY_THREE)) SetWindowOpacity(1.0f);

        BeginDrawing();
        ClearBackground(RAYWHITE);
        DrawText("test", 20, 20, 20, RED);
        EndDrawing();
    }
    CloseWindow();
    return 0;
}

[raysan5]

@asdqwe oh! nice addition! thanks!