fix(preset-mini): handle arbitrary gradients correctly

[henrikvilhelmberglund]

This PR makes arbitrary gradients in bg-[] work by returning background-image instead of background-color when linear-gradient/conic-gradient/radial-gradient are detected.

fixes #3581

before:

after:

<div class="bg-[linear-gradient(45deg,#0072ff,#00d2e8,#04fd8f,#70fd6c)] border-1 h-64 w-64 flex items-center text-center"></div>

<div class="bg-[conic-gradient(red,blue)] border-1 h-64 w-64 flex items-center text-center"></div>

<div class="bg-[radial-gradient(red,blue)] border-1 h-64 w-64 flex items-center text-center"></div>

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