fix: default variation size

If we use an icon like this, without setting up global icon preferences ```svelte <AcademicCap variation="mini" /> ``` you would expect size to be defaulted to the variation size, like viewBox, but it is not. ```html <svg xmlns="http://www.w3.org/2000/svg" role="img" width="24" height="24" fill="none" aria-label="academic cap" viewBox="0 0 20 20" stroke-width="1.5"> ... </svg> ``` This PR fixes the above issue, assuming this is not intended behaviour. <details><summary>Issue Details</summary> <br> The issue lies in these size default assignments for the different variations https://github.com/shinokada/svelte-heros-v2/blob/b1a805748a81954bc305e5996c0b8c908c593bd1/src/lib/AcademicCap.svelte#L24-L25 since size is never falsy as it is defaulted to '24' when declaring props, size is never asigned '20' https://github.com/shinokada/svelte-heros-v2/blob/b1a805748a81954bc305e5996c0b8c908c593bd1/src/lib/AcademicCap.svelte#L7-L8 </details> <details><summary>Proposal Details</summary> <br> This patch does a few things: - removes the size prop default assignments to '24' https://github.com/shinokada/svelte-heros-v2/blob/7a60c02bfe63ee7132e50461fe38ef65d33f643c/src/lib/AcademicCap.svelte#L7-L8 - instead of using an $effect, derive viewBox and variationSize from the variation prop https://github.com/shinokada/svelte-heros-v2/blob/7a60c02bfe63ee7132e50461fe38ef65d33f643c/src/lib/AcademicCap.svelte#L22-L31 - default width and height to variationSize if size isn't set (either by the consumer component or global context) https://github.com/shinokada/svelte-heros-v2/blob/7a60c02bfe63ee7132e50461fe38ef65d33f643c/src/lib/AcademicCap.svelte#L38-L39 </details>