fix: don't reassign bound imports on the server

[dummdidumm]

fixes #13027

WIP, needs a test

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[Rich-Harris]

We already treat binding to constants as an error, we should just do the same for imports so that we get a nice compile time error. Opened #13035.

If we wanted to allow binding to imports so that components (and only components, not elements) can mutate properties of a constant, then we could do that, but we should do it for all constants and not just imports.

If we omit the setter, as here, it would mean getting an error like this (depending on browser)...

Uncaught TypeError: Cannot set property blah of #<Object> which has only a getter

...which is decipherable if you know what's going on, but not totally descriptive. (The error also only happens in strict mode, though I suspect that covers everyone that uses Svelte since we're living in an ESM world.) We could provide a better error in dev, if we were to go down that road. I'm not sure it's all that necessary though.

[dummdidumm]

Closing in favor of #13035.