[@svelteui/core] Checkbox group

[BeeMargarida]

Added checkbox group component.

Decision taken:

• It differs from the Mantine approach, for example, where the checkbox instead of being passed as slots, are built by the CheckboxGroup component, since this is the only way for it to access its children props.
• Due to the existence of a known limitation of svelte in the usage of groups in checkboxes - bind:group does not work with nested components sveltejs/svelte#2308 - another approach was taken (using the on:change and managing the checked entities inside CheckboxGroup)

checkbox-group.mp4

Example Code:

<script>
       const items = [
		{
			value: "react",
			label: "React"
		},
		{
			value: "vue",
			label: "Vue"
		},
		{
			value: "svelte",
			label: "Svelte"
		}
	];
	let value = ["svelte"];
	$: console.log("value", value);
</script>

<CheckboxGroup label={"Choose your favorite framework"} description={"Choose carefuly"} bind:value={value} items={items} />
<div>Value contents: {JSON.stringify(value)}</div>

<div style="margin-top: 30px"></div>
<CheckboxGroup label={"Now with spacing"} bind:value={value} items={items} spacing={'xs'} error={"Something went wrong, oops"} />

<div style="margin-top: 30px"></div>
<CheckboxGroup label={"Now with direction 'column'"}  bind:value={value} items={items} direction={'column'}/>

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• Read the Contributing guide
• Prefix your PR title with [@svelteui/core], [@svelteui/actions], [@svelteui/motion], [@svelteui/core], [core], or [docs].
• This message body should clearly illustrate what problems it solves.

Tests

• Run the tests with npm test and lint the project with npm run lint or just run npm run repo:prepush and check to see if it's passing.