Fix svelte resolve warning in when using Vite and vite-plugin-svelte for compilation

[CHamalainen]

Hello

This is my first attempt at OpenSource contribution. This PR aims to fix a warning of deprecation/future build issues, when compiling SvelteJS apps using this package in Vite and with vite-plugin-svelte:

[vite-plugin-svelte] WARNING: The following packages use a svelte resolve configuration in package.json that has conflicting results and is going to cause problems future.

svelte-router-spa@6.0.3

Please see https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/faq.md#conflicts-in-svelte-resolve for details.

I've raised this issue a couple of months ago, and wanted to try to fix it myself: https://github.com/jorgegorka/svelte-router/issues/145

Approach:

• I have followed the suggestion from the SvelteJS documentation to add support for the exports portion of package.json, while maintaining the existing exports from the src directory. I also referred to this section of the NodeJS documentation for entrypoints for multiple submodules.
• I have tested this fork in my own app to ensure that it removes the warning.

BE AWARE: I'm unsure of whether this might remove support for other Svelte compilers (non-Vite), since it is necessary to remove the "svelte" key from package.json for the warning to go away. I was unsure of how to test / proceed with this concern. Feedback greatly appreciated.

Thank you for a great router - I hope this contribution is easy to include, else let me know if some action is needed on my part.

[jorgegorka]

Hi @CHamalainen Thank you for your PR I'll take a look at it this week.