Linting rule to prevent Markdown code fence breakage

[jeffposnick]

This is a bit of a placeholder issue, but I'll update with more details when I can confirm things.

I believe that the usage of

```js

and

```javascript

can, in some scenarios, lead to deployment issues when pushing content live to developers.google.com.

We can create a new linting rule that will detect that usage and recommend switching to

<pre class="prettyprint js">

instead.

What I'm not clear on is which scenarios will cause breakage, and should be flagged. @Meggin and @PaulKinlan has some experience with this in, e.g., #6502. Can you add your findings to this issue?