Found this while working on #12596 and didn't want it to get lost. There's no need to mess around with $$slotProps, we can just use a destructured parameter. This removes the only place where we're using state.getters in SSR mode, which means we can make that a client-side-only mechanism.

Stuff I'd like to fix while I'm at it:

• there's no need to use getters for <slot> props
• the AST is wrong — let:foo={{ bar: baz }} should create an ObjectPattern for { bar: baz }, not an ObjectExpression. Minor point but should be easy to fix actually it would be annoying to fix, and we'd have to undo the fix for the legacy AST. we're going to get rid of it one day anyway so let's just leave it

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