feat: pure version of separating js and css

[Nomango]

Summary

As discussed in #203, provide a JS bundle without CSS injection and a separate CSS file.

fix #203.

Result

Add two outputs for rollup, generating the following files.

$ tree ./dist                                                          
 ./dist
 ├── medium-zoom.d.ts
 ├── medium-zoom.esm.js
 ├── medium-zoom.js
 ├── medium-zoom.min.js
+└── pure
+   ├── index.d.ts
+   ├── index.js
+   └── style.css

1 directory, 7 files

Usage:

// import pure js files
import mediumZoom from "medium-zoom/dist/pure";

// import pure style files
import "medium-zoom/dist/pure/style.css";

!!! Tested in Astro only. !!!

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Latest deployment of this branch, based on commit 54c1f65e150136719f7ae6da6e22dedc43a82637:

Sandbox	Source
@medium-zoom/vanilla-example	Configuration
@medium-zoom/vanilla-example	Issue #203

[francoischalifour]

Thanks so much for handling this @Nomango. I'll review this in the next days.

[francoischalifour]

Changing the base branch to apply further changes to this PR.