Better autocompletion

[Rich-Harris]

We don't have the full power of the TypeScript language server, but we can probably offer some useful autocompletions:

• import sources
• {#each, etc
• data-sveltekit- attributes
• on: directives

[Rich-Harris]

In fact even without the language server we could still probably provide autocompletions for things like array methods if they were declared in scope...