[rcore] Add GetMonitorScale()

[ghost]

Ref: #4522

Necessary to allow the user to factor in arbitrary user defined/applied monitor scales (GLFW ref, SDL3 ref, Web ref).
Note: this is different from GetWindowScaleDPI (GLFW ref, SDL3 ref).
The PR can be tested with:

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

        const Vector2 monitorScale = GetMonitorScale(GetCurrentMonitor());

        BeginDrawing();
        ClearBackground(RAYWHITE);
        DrawText(TextFormat("monitor scale %fx%f", monitorScale.x, monitorScale.y), 20, 20, 20, BLACK);
        EndDrawing();
    }
    CloseWindow();
    return 0;
}

[ghost]

After further review GetMonitorScale() will basically end up returning the same as GetWindowScaleDPI(), so I guess no need for this function afterall.