[@svelteui/core]: add Switch innerLabelSize property

[xon52]

Description

While using inner labels (onLabel and offLabel) for the Switch component, I noticed that text was often moved further out of frame the longer it was, and that the size of font was unsuitable for emojies:

To fix this I added a new property to override onLabel and offLabel sizes called innerLabelSize. I also changed the onLabel CSS from transform: translateX(-200%) to left: 10% which mirrors the offLabel CSS of right: 10%.

Would love some feedback as my first contribution to this repo. Not too fussed if it's not suitable; it fixes my issue and thought I would share.

Before submitting the PR, please make sure you do the following

• [x] Read the Contributing guide
• [x] Prefix your PR title with [@svelteui/core], [@svelteui/actions], [@svelteui/motion], [@svelteui/core], [core], or [docs].
• [x] Include a description of the changes made in the PR description and in the commit messages.
• [x] Include screenshots/videos of the changes made.
• [x] Verify that the linter and tests are passing, with yarn lint and yarn test or just run yarn prepush.

[BeeMargarida]

Sorry for taking so long to answer, this got buried in my notification box! I'll review this today!