feat: add support of arbitrary mounting point via attachTo option

[jnields]

This allows for users to specify where in the document their component should attach, either through
a CSS selector string or a provided HTMLElement. This option is passed through directly to the
vm.$mount method that is called as part of mount.js

feat #1487

This enables testing of SSR code with Vue test utils as well as enables rendering of applications via Vue test utils in contexts that aren't 100% Vue

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch.
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue-test-utils/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[jnields]

I added in a message that attachToDocument would be deprecated in the future. Let me know if you'd like me to remove it or update any messaging.

I also didn't add any checks to see if both attachTo and attachToDocument were provided. I can add such a check and messaging for that if you think it's necessary.

Also happy to help with any documentation updates that might be required. Please let me know.

Thanks!

[lmiller1990]

Hi! Thanks for the PR.

Seems reasonable. Happy to merge this... but can you please update the docs to include this? Otherwise no-one will be able to use this new feature!

[lmiller1990]

@jnields do you think you will have some time to update the docs by any chance? Going to do a 1.0 release pretty soon - would be good to get this in.

[jnields]

@lmiller1990 awesome, thank you! I pushed up some doc updates in English.

[lmiller1990]

Looks good. This will go out with 1.0. We are finalizing the API. Aiming to have something out within a month.