feat(measurements): Add ‘hm’ (himetric) unit

Metric equivalent of ‘twip’ in the SI world, adding largely for
posterity because it's low cost. I use mostly imperial units, but this
could be useful for the same reason twips are used instead of very small
decimals of points: working with smaller units and larger numbers is
easier mental gymnastics than very small fractions of large units.

core/units.lua

@@ -57,6 +57,10 @@ units["m"] = {
   definition = "100cm"
 }
 
+units["hm"] = {
+  definition = "0.01mm"
+}
+
 units["in"] = {
   definition = "72pt"
 }