fix: use init properties when exporting non-state values

[Rich-Harris]

Follow-up to #10520. Changes this...

var $$accessors = {
  get message() {
    return message;
  }
};

...to this, for non-state values (except in dev, where we want to guard against people incorrectly setting stuff from outside):

var $$accessors = { message };

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Latest commit: 22db33e

The changes in this PR will be included in the next version bump.

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