feat(StackBlitz): Add support for StackBlitz

[crutchcorn]

What: StackBlitz is being added to the list of supported vendors

Why: Many sites may use StackBlitz to embed usage of code samples into their blog. I run one such site myself, and intend to use this plugin once these changes are merged

How: We implement a regex for the functionality behind the embed like other services do

Checklist:

• Documentation
• Tests*
• Ready to be merged
  

I will be adding documentation once the tests are finally passing - I am asking for help debugging these tests

I am making a WIP PR as I feel that I've ran into a bug somewhere along the line with the tests I wrote. I keep running into the following error:

 FAIL  src/__tests__/transformers/StackBlitz.js
  ● Plugin can transform StackBlitz links

    expect(received).toMatchInlineSnapshot(snapshot)

    Snapshot name: `Plugin can transform StackBlitz links 1`

    - Snapshot  - 1
    + Received  + 1

      <p>https://not-a-stackblitz-url.com</p>
      <p>https://this-is-not-stackblitz.com</p>
      <p>https://this-is-not-stackblitz.com/edit/start-to-source-1-ng-template</p>
      <iframe src="https://stackblitz.com/edit/start-to-source-1-ng-template?embed=1" style="width:100%; height:500px; border:0; border-radius: 4px; overflow:hidden;" sandbox="allow-modals allow-forms allow-popups allow-scripts allow-same-origin"></iframe>
    - <iframe src="https://stackblitz.com/edit/start-to-source-1-ng-template?embed=1&file=src/app/app.component.ts" style="width:100%; height:500px; border:0; border-radius: 4px; overflow:hidden;" sandbox="allow-modals allow-forms allow-popups allow-scripts allow-same-origin"></iframe>
    + <iframe src="https://stackblitz.com/edit/start-to-source-1-ng-template?embed=1&#x26;file=src/app/app.component.ts" style="width:100%; height:500px; border:0; border-radius: 4px; overflow:hidden;" sandbox="allow-modals allow-forms allow-popups allow-scripts allow-same-origin"></iframe>

Notice the "expected" embed=1& vs the "received" embed=1&#x26

I'm not sure why this is happening and I'm not entirely confident that this issue is a result of my code - I'm asking for help figuring this one out 😅

Once initial support for StackBlitz is added, I intend to add further functionality by adding support for optional configuration for StackBlitz embeds

[crutchcorn]

@MichaelDeBoey was wondering if you'd had a chance to review?

I'm still admittedly unclear why tests are failing, as I don't see anything that I should necessarily change