Add Spacing [SPAC] axis (#62)

* Add TRAK custom axis

* Update and rename tracking.textproto to spacing.textproto

* Update spacing.textproto to add decimals to vals

To make the defined precision value more explicit.

Similar to a133509583fa8e53f15684028cdc1bb74a6039ee

* Renaming fallback to "Default"

* Update spacing.tp with reference project URL

* Update spacing.textproto to revert fallback name

* Update Spac description 

Following agreed decisions on meeting

* typo fix

Co-authored-by: Dave Crossland <dave@lab6.com>
Co-authored-by: Viviana Monsalve <viviana.monsalve.a@gmail.com>

Lib/axisregistry/data/spacing.textproto

@@ -0,0 +1,16 @@
+# SPAC based on https://github.com/gferreira/roboto-flex-spac
+tag: "SPAC"
+display_name: "Spacing"
+min_value: -100.0
+default_value: 0.0
+max_value: 100.0
+precision: -1
+fallback {
+  name: "Default"
+  value: 0
+}
+fallback_only: false
+description:
+  "Adjusts the overall letter spacing of a font. "
+  "The range is a relative percentage change from "
+  "the family’s default spacing, so the default value is 0."