Vue2: Fix play function within & args updating in decorators

[shilman]

Issue: #19204

There are a couple problems with Vue2's renderToDom

• The element #storybook-root is re-created on first render, which breaks the play function's within(canvas)
• Decorators that reassign args, e.g. storyFn({ args: { somethingElse } }) don't work like the other renderers. Unfortunately this is a pattern we use in our sandbox Pre component all over the place.

What I did

Fixes with @prashantpalikhe:

• Render the story into the child of #storybook-root instead of overwriting it
• Fix args handling so that args can be updated by decorators
• Enable CI for vue-cli/vue2 repro template

How to test

• CI passes

[IanVS]

Does this PR address #19204?

[shilman]

@IanVS Not yet. This PR addresses within(canvas) failing in play functions. It's kind of a hack so I'd like to work with somebody to figure out the "args binding" problem described in that comment before merging anything

[shilman]

@IanVS update: i looked at this with @prashantpalikhe and we're closer to a solution but still have more to figure out. thanks for your patience!