Deprecate `config.kit.files.*`

[Rich-Harris]

Describe the problem

For reasons that are lost to the mists of time, we allow you to configure the location of src/routes, src/lib and so on. It adds complexity and inconsistency for no good reason.

Having the structure of your app match what's in the docs is an unambiguously good thing. This was always true (the whole point of a framework is that you're able to easily move between codebases), but it's even more true in a world of coding assistants.

Describe the proposed solution

Deprecate all the config.kit.files.* options

[benmccann]

For reasons that are lost to the mists of time, we allow you to configure the location of src/routes, src/lib and so on

Maybe it's a holdover from before we even used Vite? #193. In any case, I'm still not sure why we did it 😆

I found a surprising number of people using the option to set the default value:
https://github.com/snuffyDev/Beatbump/blob/d00ed3e10a13ec82a6a006dd02bf9548544d1ab3/app/svelte.config.js#L52

@jhipster seems to be using it somehow to place the a Svelte project inside a Java project. @jhipster perhaps you could expand upon why you need to do this and if losing the option would cause you difficulty?
https://github.com/jdubois/jhipster-lite/blob/6a8fcd4cdb7b5999ba3a940ef3bd34ae7321ff91/src/main/resources/generator/client/svelte/svelte.config.js.mustache#L15
https://github.com/jhipster/generator-jhipster-svelte/blob/aa64f0c2a6f5a59f5a71b8e6f47001c5a4749e50/generators/client/templates/svelte.config.js.ejs#L21

I also see eslint-plugin-svelte using these options, so have opened an issue there (sveltejs/eslint-plugin-svelte#1294) in case @baseballyama can help us remove use of those options or let us know why they're needed

I also found one that's kind of a gross workaround for not wanting to setup a monorepo: https://github.com/mquandalle/mesaidesvelo/blob/c8fdd949480cdd27e32b8b70d2f13065ae296490/svelte.config.js#L11

And one where they just wanted to have the svelte.config.js file live somewhere different from the source: https://github.com/stemrollerapp/stemroller/blob/main/svelte.config.js