[feat] add share button in REPL

[johannchopin]

Fixes #95.

The button svg was created by using figma https://www.figma.com/file/9F50hFRCB9jdKgSvAdGNG7/svelte-edit-repl-btn?node-id=0%3A1. If a modification is needed just tell me I will add you as a collaborator.

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[johannchopin]

@benmccann Do you have suggestion to clarify the term embed over share?

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[benmccann]

Here's the original PR for reference for the other reviewers: sveltejs/svelte#6703

[Rich-Harris]

Thank you! I'm not sure it makes sense to add this to the UI until (/unless) we have a mechanism for creating embeds. We'd need to have a thorough conversation about whether that's something we want to do, and what it would involve exactly. In the meantime I think links are the appropriate way to share REPLs