Add support to highlight single lines of code

[bdemers]

It would be great if we could bold/highlight single (or a range of lines) of code in our snippets in order to draw attention a specific bit of code in a block:
https://vuepress.vuejs.org/guide/markdown.html#line-highlighting-in-code-blocks

This looks like it should be possible today, but I wasn't able to get it to work locally.
I'm guessing it's related to our use of markdown-it-attrs:
https://github.com/okta/okta-developer-docs/blob/master/packages/%40okta/vuepress-site/.vuepress/config.js#L286

I tried a few things without any success:

• Updating the version of markdown-it
• Adding a separate plugin: markdown-it-highlight-lines

NOTE: the Vuepress 2.x (beta) also mentioned they forked the highlight-lines plugin, so if we are planning on upgrading sometime after 2.x is released, then it's possible this could just work.

TL;DR: Highlighting, a single line (or lines) of markdown should work via something like:

```js{2-3}
console.log("dull code")
console.log("cool code")
console.log("more cool code")
console.log("other dull code")
```

[janethu-okta]

Hi Brian,
Create a JIRA ticket for the team to look into your suggestion.
https://oktainc.atlassian.net/browse/OKTA-485123