[feat] merge packaging exports

[ignatiusmb]

Resolves #1944

Still not sure if package.exports.behavior is a good settings, inputs are welcome. Also note to self, need to add some docs.

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[dummdidumm]

I think this is a good addition. I'm split on whether we should do a boolean (that could become a function) or do the enum. If there's more than true/false/function, then definitely the enum, but other than deepMerge I don't see a valid third option (and maybe merge should merge deep anyway).

[ignatiusmb]

Come to think of it, there can't be a situation where both behavior is set to replace and include/exclude are also used. What if we set the default value of kit.package.exports to null, so exports map in package.json wouldn't get tampered by SvelteKit when user has defined it by themselves, but if it's empty, it will use the generated version.

One awkward thing to this is that users that would like to get their self-defined exports map merged but still have all other exports generated would need to pass an empty object to the config, exports: {}, to indicate they're not replacing the generated ones. But, this allows us to keep the config as is, any thoughts @benmccann @dummdidumm?

[dummdidumm]

I'm not completely sure I understand. Rereading #1944, there's truth in the OP's post: " if I don't want svelte-kit package to touch my exports I can just do exports:{ exclude: ["**"] }." So maybe the best way would be to just skip the extra layer of "is this null" or "add a behavior param" and instead request users to configure their include/exclude correctly. The new default behavior would be to merge everything present in package.json - why else would you add exports there if you want them overwritten?

[ignatiusmb]

What would I do without you and your reviews @dummdidumm

[dummdidumm]

Well I didn't get to this in hindsight obvious conclusion, too, at first 😄