Add an external link icon and css style.

[StevenDufresne]

External links should display an external link icon. This PR adds an svg, taken from Gutenberg, and a class to target external links.

I wasn't exactly sure what color to give the icon. I went with a moderately dark grey color from our theme.

See WordPress/wporg-showcase-2022#42.

Drawback of this approach

Since we are only using CSS and not a block or something similar, users cannot see the external icon when viewing the post in the block editor. However, I don't expect there to be many external links, and doing more work should probably be considered only if we receive feedback from teams (maybe the documentation team may find that frustrating for example).

Screenshots

Caption	Screenshot
Default
With Element font-size: 40px
Inline
Alternative color option -- Not Implemented here

How to test the changes in this Pull Request:

• Add a link to some post content, add external-link class to the element. Expect to the the icon.
• Alternatively, you can add the class to the direct parent of the <a>
  • This is done in case we don't have direct access to the <a> element.

[StevenDufresne]

Here's the updated view:

Place	Image
Regular link
Scales with font size
Embedded link
Embedded link light

Notes:

• I have added a external-link-light for places where we have dark backgrounds
• I have also added styles in case we have access to the anchor tag directly

[ryelle]

Looks good 👍🏻