Stylo: Implement font-kerning

Please go through the guide before working on this issue.

font-kerning is a keyword property which takes one of the following values: auto | normal | none. The default value is auto. The linked MDN has more information about this.

The constant prefix you should be using is NS_FONT_KERNING

You will need to manually implement the glue, see https://github.com/servo/servo/pull/13570/files for an example of this.

Code: servo/components/style/properties/longhand/font.mako.rs

• Use the example from the MDN page.
• Build stylo and screenshot how the example looks before your changes.
• Make your changes, build stylo and screenshot how it looks after.
• Open a PR with your changes and attach the screenshots

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Please make a comment here if you intend to work on this issue. Thank you!

[Manishearth]

Make sure the MDN example has visible kerning. You may need to tweak the font to get this to work. If the MDN example shows no kerning difference between the two example lines, try the following HTML example:

<style type="text/css">

div {
  font-size: 2rem;
  font-family: arial;
}

#nokern {font-kerning: none;}
#kern {font-kerning: normal;}
</style>

<div id=kern>
AV T. ij
</div>

<div id=nokern>
AV T. ij
</div>