chore: unified expression metadata

[Rich-Harris]

Another step towards the more comprehensive compile-time analysis of each block dependencies that will unblock #12511.

Currently, various nodes need to know whether the expressions they contain are dynamic (i.e. reference state, or could reference state) and/or contain call expressions (which may mean they need to be wrapped in a derived, or a separate effect or whatever). This happens in a slightly ad hoc and messy fashion.

This PR neatens it up, and also adds a dependencies set to each expression metadata object, which should soon enable more detailed analysis. The goal is to be able to optimise cases like this...

{#each [1, 2, 3] as n}...{/each}

...where it's frivolous to wrap the array members in signals.

There is a blocker to doing that currently: we would need to call context.visit(node.expression, ...) inside the EachBlock visitor. That's not possible, because the analysis-phase visitor merging prevents us from calling context.visit. I've been pissed off at this for a long time, and this is the straw that broke the camel's back — it's time to overhaul the analysis phase and have a single visitor per node type. Going to work on that as soon as this is merged.

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[dummdidumm]

Two small things, otherwise 👍