Prepare for 0.6.0 release

[nicoburns]

Bump version numbers for 0.6.0

[DJMcNab]

I've not been carefully keeping up with Parley, so I'm not best placed to evaluate whether we need a release, if the changelog is up to scratch, etc. I'll be happy to do the process once this is approved, though.

Before we land this, I'd like to see Masonry and Xilem brought up to the latest main here, as a smoke test. Based on the changelog, I believe that should be trivial, but if not, then the changelog probably needs to be updated.

(There's also a lot of malformed formatting in the changelog, incidentally, which should be fixed)

[nicoburns]

I've not been carefully keeping up with Parley, so I'm not best placed to evaluate whether we need a release

The primary motivation for a release is the removal of the kurbo/peniko dependencies. This will allow a new release to play nicely with a new Vello release. It will also be very nice to get a release out with the HarfRust changes: that will mean we finally have correct shaping. And some of the optimisations mean that Parley main is also quite a bit faster than 0.5.

Hopefully the removal of kurbo/peniko in this release will mean that we don't necessarily have to coordinate (breaking) Parley releases with (breaking) Vello releases in future.