Feature/support for fragments

[przemkow]

Hi! After @sodatea 's tweet, I decided to try my first contribution to Vuejs project :) This PR is based on the previous implementation of vue/valid-template-root if you think that something is missing or should be changed I will try to do it as soon as possible!

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Add support for Vue 3 fragments.

Relates to:
#1035

Scope:

• vue/valid-template-root: Remove a single root element check. New implementation checks two different cases:

  • When <template> does not have src attribute, it needs to contain at least one element
  • When <template> have src attribute it needs to be empty

• vue/no-multiple-template-root: New rule-based on previous vue/valid-template-root implementation used to check if template contains single root element (implemented for Vue 2 backward compatibility)

Clarification needed:
How can we differentiate "essential" rules for Vue 2 from the one for Vue 3? Currently vue/no-multiple-template-root is added to configs/essential.js but I'm not sure how to do it correctly to do not affect build for Vue 3.

[ota-meshi]

Sorry for the delay, looks good to me! Thank you!

[ota-meshi]

I think creating categories for Vue3 is a separate task.