Setup metacrate

Currently using Skrifa and HarfRust is a pita. Resolving this has two main steps:

1. Make some of the types common (out of scope here)
1. Make it easy to pick versions that have the same version of major dependencies, most notably the fontations crates such as read-fonts

This repo is intended to house a metacrate that lets users select which of {Skrifa, HarfRust, Klippa, more in future} they wish to use and guarrantees that when used via the metacrate only one version of dependencies will be pulled.

CI (or other automation) should ensure the metacrate cannot exist in an invalid state, such as by inspecting `cargo tree` for many instances of the same dependency with inconsistent version.