Add spacing/tracking variable axis about shantell-sans

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Okay, so I’ve done some experiments in the tracking-axis branch, and things are looking pretty good.

The key: a "tracking" axis can be added with just a single source, if that source is otherwise at the default location.

Example:

• A Weight + Tracking designspace

<axis tag="wght" name="Weight" minimum="300" maximum="800" default="300"/>
<axis tag="TRAK" name="Tracking" minimum="0" maximum="1000" default="0"/>

• There is a Light source and an ExtraBold source for weight.
• If we make a Light Tracked source, we can have tracking without doubling the masters. E.g. "ExtraBold Tracked" can be arrived at by adding up the deltas from the defaults.
• If we instead try to use only an "ExtraBold Tracked" source, we cannot get the Light Tracked result.

Here’s a demo:

Kapture.2022-02-26.at.18.36.18.mp4

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Next steps:

• Getting the 4 primary sources compatible again, after all the additional glyphs
• Integrating this UFO tracking script into the build prep and designspaces

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The next step will be to get this into the GF axis registry.

Relevant links:

fonttools/fonttools#2564

googlefonts/axisregistry#27