Full migration to Vue 3 (and also Vite)

[11BelowStudio]

Incompatible with #4 and is an updated version of #5

I have managed to fully migrate this template from using Vue 2 to Vue 3, along with using Vite instead of Vue-cli for deployment and such.

The functionality of the template is unchanged.

Of course, the linter now complains about a few more things (such as the single-word component names in general, the names of Header.vue and Footer.vue being "Header" and "Footer" (HTML words), and a couple of typescript linter complaints about code which was there already and still works) - but as changing some of them would be breaking changes, I figured I may as well avoid those breaking changes, just to make things easier for everyone.

Also the readme has been updated with Vite instructions instead of Vue-cli instructions.