fix(utils): try to improve mobile detection

[dawedawe]

Comes with its own set of drawbacks but maybe still a bit better than using the size 🤷

[orhun]

Hey, nice!

Some questions:

• Did you run into an issue where the mobile isn't detected with the previous approach?
• What kind of fallbacks does this have?

[dawedawe]

Hey man, thanks for the review.

* Did you run into an issue where the mobile isn't detected with the previous approach?

If you use a mobile browser and just change the device orientation to landscape, the old approach would claim the device is not a mobile anymore. To me, the function is_mobile should really try to determine the device class and leave questions about the size to the size functions like get_raw_screen_size that the ratzilla programmer can then interpret according to their requirements.
In general, keeping the API a bit more orthogonal here seems good to me.

* What kind of fallbacks does this have?

The drawbacks of using the agent string are:

• Users can set the user agent of their browser to any string
• Browsers can change their user agent from one version to another

I think these drawbacks are acceptable, we just have to be aware of them.