feat!: minimize package structure

[metonym]

The current, published NPM package structure resembles the following:

src/*
package.json

This PR revises the structure to be even shallower:

package.json
index.js
Time.svelte
...

The resulting breaking change is:

- import Time from "svelte-time/src/Time.svelte";
+ import Time from "svelte-time/Time.svelte";