Update the note about Vue Router and lazy loading

[skirtles-code]

This is inspired by the change proposed in #1869.

In Vue 2, Vue Router used Async Components for the lazy loading of route components. However, in Vue 3 these are two similar but distinct features. The description in performance.md hasn't taken this separation into account.

The main change is to the paragraph describing the use of lazy loading with Vue Router. This has been updated to reflect the separation between these two features.

I've also tweaked an earlier paragraph to remove the word 'typically'. I think that was a little misleading in a Vue 3 context, as lazy loaded routes are more typical than explicitly using defineAsyncComponent.

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