feat: support relative import inside slides

[kermanx]

This PR is based on #1708. It will be much smaller after merging that one.

Previously, it wasn't possible to import assets via relative paths. For example, ![](./assets/a.png). This was because the slides contents were loaded as /@slidev/slide/1.md.

This PR loads the slides in path/to/slides.md as path/to/slides.md__slidev_x.md, where x is the slide no in the presentation, instead of the slide no in that Markdown file. And /@slidev/slide/x.md works as a facade which re-exports path/to/slides.md__slidev_x.md. The same for frontmatter.

The reason why not use urls like path/to/slides.md?slidev=x.md is that this will conflict with Vue's logic. The Vue plugin doesn't preserve the query of the URL. Also, Vue plugin's include option only matches the path part without query string.

This PR also splits vite/loaders.ts into multiple files - each Vite plugin has its own file. And removed most of the usages of enforce/order.

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[antfu]

Thanks for working on it! You are amazing!

[antfu]

I am ok with the direction, I'll let you handle the merge and release