feat: dynamic route animation

[stefanholzapfel]

Description

I needed different behaviour for my route animations, depending on the route the app is going to. E.g. components should leave to the left when going deeper in the routing tree, leaving right when going "back". The simplest way was to enable the router to accept a factory function for the "animate" property that would be evaluated on every route change.

The "animation" property now also accepts a function that returns the animation object:

animate: boolean | { enter: string, leave: string} | (from: HTMLElement, to: HTMLElement) => { enter: string, leave: string }

closes #845

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.

Additional for Feature type of change

• Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.

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[stefanholzapfel]

Hi @emarc @pdurbin!
Any chance we can merge this? Thanks!