add more sourcemap mappings

[Rich-Harris]

WIP — want to add other nodes, and also verify that this works as intended (which is hard to do by eyeballing the generated sourcemaps)

[changeset-bot]

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Latest commit: 32bb8a6

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[Rich-Harris]

Tested this and it doesn't really change anything — was hoping it would give the browser something to 'latch on to' when trying to find the source of a change.

It's still a minor improvement when looking at stuff in the Svelte playground though, and there's no real reason not to do it, so... I'll do it