Custom path for shared helpers/methods

[Rich-Harris]

Right now the path for helpers and methods is svelte/shared.js. Would be good to make this customizable so that e.g. rollup-plugin-svelte can resolve the path itself, without the app developer needing to use rollup-plugin-node-resolve

[Rich-Harris]

In fact, thinking further on this, perhaps the solution to this is to have a shared option instead of options.standalone, where shared: true is equivalent to shared: 'svelte/shared.js', and shared: false is equivalent to standalone: true.

[Ryuno-Ki]

Hm, I wonder what would happen if I set options.standalone to a value different to true and shared:false …

[nikku]

options.standalone is gonna be replaced with options.shared.