feat(design)!: change the diameter property of loading icon to a string instead of number

[xelaint]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/graycoreio/daffodil/blob/develop/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

The diameter property currently takes a number. This means the number is always a fixed value, currently set in pixel.

Fixes: N/A

What is the new behavior?

Change diameter property to accept a string so the diameter number can pixel, rem, etc.

Does this PR introduce a breaking change?

[x] Yes
[ ] No

BREAKING CHANGE: The diameter property is now a string and the CSS units need to be specified.

Other information