feat: better autocomplete

[Rich-Harris]

This scratches a personal itch with the preview site.

Our autocomplete is somewhat buggy:

• it suggests runes inside strings and comments
• it suggests nothing for e.g. $effect.
• for $effect.p it suggests $effect.pre, but then submits $effect$effect.pre(() => {...})
• it has no awareness of context — it will suggest things like $state in any position, when in reality it is only valid in very specific positions

This PR fixes all those things. It also adds autocomplete for import declarations (though just the module IDs, for now — I feel like if we start adding proper autoimports we'll end up jankily reimplementing the TS language server).

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Latest commit: 0e79e07

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