feat: Add createStubs plugin hook

[freakzlike]

Adds a new plugin API to config.plugins called createStubs which will override the default stub creation provided by VTU.

Usage:

config.plugins.createStubs = ({ name, component }) => {
  return defineComponent({
    render: () => h(`custom-${name}-stub`)
  })
}

When VTU creates a stub either from shallow: true or stubs: { ChildComponent: true }, then the provided function will be called.

This will allow to implement different custom logics which are mentioned here:

• Stubbed components slots aren't been rendered when 2 deep. #773 (comment)
• Stubs rendering default props #945
• feat: Hide props with default values within snapshot #1174
• Extend stub via mounting options #1179
• Mounting only the slots of stubbed child components #69

I have not updated the docs yet, but I will if it is good.

Closes #1216

[cexbrayat]

This might be a good idea. @lmiller1990 WDYT?

[lmiller1990]

Cool, but isn't this exactly the same as

import { config } from '@vue/test-utils'
config.globals.stubs = {
  // your stub
}

Maybe I did not read it properly or misunderstood - don't we already have the ability to create a stub globally like this?

Also, if we have a new feature, we should definitely document it, how it works, and why it's useful. This might also be useful for people reviewing it (like me :D )

[freakzlike]

Ok, I will add the docs soon :)

It is not used to stub one specific global component. Instead this will be called for all stubs so something similar like this:

import { config } from '@vue/test-utils'
config.globals.stubs = {
  '*': GlobalStub
}

Also the generated stub can by generic depending on the component which should be stubbed

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[lmiller1990]

I guess we can have this, although I'm not sure how much it'll be used.

I'd say this should be last feature before we move to 2.0 stable.