fix(preset-icons): add custom icon handling without unit - #5279

close #4084, close #4060, close #4089

When iconify/utils parses SVG tags, if the width and height attributes exist in the tag, it will automatically add them to the props without performing any operations on the values.

<svg width='32' height='32' xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256"></svg>

Will parsed:

{
  width: 32,
  height: 32,
}

If neither the width nor the height is present, iconify/utils will automatically parse or calculate the aspect ratio and add it using the default unit (em).

<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256"></svg>

Will parsed:

{
  width: 1em,
  height: 1em,
}

However, for UnoCSS output CSS styles, sizes without units are not considered valid.

This PR will handle this uniformly in the final parsed results, and will correct any invalid units.