text: support fenced code blocks in Markdown

[cghawthorne]

• Motivation for features / changes

My particular use case is dumping a json config file, but the fenced code syntax is pretty common in markdown and I suspect it will have lots of uses.

https://www.markdownguide.org/extended-syntax/#fenced-code-blocks

• Technical description of changes

Just adds the fenced_code extension.

• Screenshots of UI changes

• Detailed steps to verify changes work correctly (as executed by you)

Log a text event with a fenced code block, verify it renders correctly.

• Alternate designs / implementations considered

I don't know another good way to include a block of preformatted text.

[wchargin]

Thanks for the PR! This seems pretty reasonable to me. Let me take a
quick check to make sure that this works internally.

I don't know another good way to include a block of preformatted text.

You can prepend four spaces to each line, thus:

```
I am
a code block.
```

I am normal text.

    I am also
    a code block.

But fenced code blocks are often more convenient, to be sure.

[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

ℹ️ Googlers: Go here for more info.

[wchargin]

@googlebot I consent.

[wchargin]

Thanks!

[wchargin]

(Test sync passes: http://cl/352909055.)