Add support for Lit-style lowercase props

[patricknelson]

For feature parity with Svelte 4.0 (and simply to enable the availability of uppercased props in custom element attributes), we should standardize on Lit-style attributes, per sveltejs/svelte#8457 (emphasis mine):

... and what the corresponding attribute for the property is (attribute, default lowercased prop name). These options are heavily inspired by lit: https://lit.dev/docs/components/properties.

e.g.

<custom-element foobar="baz></custom-element>

Where the element attribute foobar is automatically translated to its camelCase counterpart: fooBar, making it available in CustomElement.svelte as:

<script>
  export let fooBar = '';
</script>

<div>{fooBar}</div>

This should supersede the prior request to support kebab-style to camelCase conversions (issue #11), since that is apparently not the direction that Svelte 4 will be going. Sticking with Svelte's choice should make it easier for someone to migrate, if necessary. Just setting up this issue so I can track it and implement it when able.