Compile error for sidebar

[lmaccherone]

Similar to this issue, I've followed the instructions for the sidebar to perfection, and I'm getting this compile error. If I move the declaration down into the constructor, the error switches to the next line in context.svelte that also has the same problem.

src/lib/components/ui/sidebar/context.svelte.ts:6:16 "$derived.by(...)" can only be used as a variable declaration initializer or a class field

I've tried both things suggested by the linked issue: upgrading bits-ui (it was already at 1.3.13, but I downgraded to 1.3.5) and I was already at a mixture of ES2022 and ESNext.

Note, I'm using vite + svelte and not SvelteKit if that makes a difference.