Take DPI into account when requesting fixed size images

[kara]

To reproduce:

1. Add nuxt/image to modules (in config, yarn add, etc)
2. Add nuxt/image instance with width and height set

<nuxt-img src="cat.jpg" width="300" height="200" />

Expected behavior:
Image component has a srcset attribute that defines a different image width for 1x and 2x images. On a laptop with 2x DPI, the image should ultimately be requested at 600px (2x DPI x given width).

Expected result:

<img src="cat.jpg" srcset="cat.jpg?w=300 1x, cat.jpg?w=600 2x" />

Actual behavior:
There is no srcset attribute generated. On a laptop with 2x DPI, the image is requested at 300px instead of 600px. As a result, the image produced looks grainy.

Actual result:

<img src="cat.jpg?w=300&h=200" />

Suggested change:
The image component should take the user's DPI into account when requesting images by defining a srcset with density descriptors. Given that mobile devices can have unnecessarily high DPI (e.g. 3x), it may be optimal to cap the DPI at 2x.

[hartmut-co-uk]

ref / ~duplicate of #618

[kara]

Closed by #769