To complete the existing function layer_property(...), this change add a new argument 'distance_units' for return a string with the layer distance units (see QgsUnitTypes::DistanceUnit)
This function can be used for display units for labels, in layouts or for access to more layer properties in the expression builder for algorithms.
A new script then parses these json help files into the html help
contents, in a standardised manner. Previously, the function help
was a bunch of different formats and structures.
Now, a custom stylesheet can be applied (and later tweaked!) to
format these help. Eg, this commit colors variables and function
names in the syntax section for greater readability.
