Extend @webstone/cli to wrap svelte-add

[mikenikles]

The svelte-add project (repo link) is a convenient way to configure various things in SvelteKit apps, e.g. Tailwind CSS, mdsvex, etc.

The Webstone CLI should provide a command which acts as a thin wrapper around svelte-add. For example, webstone web configure tailwind is a wrapper for npx svelte-add@latest tailwindcss (docs).

Update: Since this is the first CLI command, let's also set us up for success as we add more commands. See best practices docs.

[mikenikles]

The Webstone CLI wrapper command is webstone web svelte-add <integration>. This makes it clear we delegate to svelte-add and gives us an opportunity to use webstone web configure for other, Webstone-specific configuration tasks.