Publish tooling to generate font from sources

[davidjohnoliver]

What would you like to be added:

At the moment, only the font itself is visible. We should also publish the automated tooling to generate the Uno font from source icon fonts, based on a spreadsheet of mappings of glyph position to source icon.

Why is this needed:

Allow new mappings to be contributed by the community (with the aim of covering as much of the Segoe MDL2 icon range as possible using the Fluent System Icons source).

Anything else we need to know?

See unoplatform/uno#967

[robloo]

Information should also be generated to show the source of each icon: Which Icon pack it comes from, license, whether it was created only for Uno, etc. This will help manage customizations in the future.

[davidjohnoliver]

Note for contributors:
The base Fluent System Icons are now available as fonts, whereas at the time Uno.Fonts was initially released they were only individual SVG files.

The Fluent System Icons font can't be used as a drop-in replacement, because the glyph positions don't align with Segoe UI; but it should be more straightforward now to develop a conversion tool (based on eg @kakone's https://github.com/kakone/SvgFont2Files), to automatically generate the Uno symbol font from a version-control-friendly spreadsheet.