Adds minimal dev/demo server

[ozanozbek]

• Adds a minimal demo server for development and demonstration purposes.
• All new files are added from official svelte app template.
• rollup configuration for demo is kept in new rollup.demo.config.js file.
• /demo folder contains static demo files.
• /demo/build is populated by output files.
• /src/demo is added to include demo in development.
• demo and dev scripts are added.

[antony]

Thanks for this, but closing in favour of SvelteKit which has a proper dev server / demo / hmr set-up. This would add complexity to this repo, which would lessen the usefulness as a simple bundler example.