Add changelog for 1.9.0

[straight-shoota]

Adds the changelog for 1.9.0 in a new format as proposed in #13620.

Preview

The new format is a proposal and not yet confirmed for 1.9. But I hope it will because at least I think it's much better than the old one :-)

[straight-shoota]

A short comment on the breaking changes:

Most of them (#13293, #13294, #13350, #13554) are related to bugfixes for faulty comparisons between Big number types and float special values (such as NaN). They used to raise SIGFPIPE which would typically crash the program. The fix introduces Nil into the return types of several <=> methods. This mean that code which does not expect the type to be nilable will fail to build.
Unfortunately this is a necessary evil as there is no sane way to fix this serious bug without API breakage.

#13546 also fixes a bug. In this case the breaking only affects code which expects the incorrect behaviour of YAML::Any to parse "yes" as bool instead of string. Which is very unlikely. It's debatable if this should even be considered a breaking change.

#13448 is a bit different. IIUC this is mainly motivated by removing legacy code from our custom LLVM extension and cleaning up the bindings. It's not strictily necessary. Our extension functions were a bit different than the (now available) official API and these differences now propagate to the tidied up stdlib LLVM API. Since these extensions are primarily intended for the Crystal compiler itself, breaking the API should not have a big impact. In fact it now makes it easier to use these LLVM features without needing the extension library.

Related: https://forum.crystal-lang.org/t/breaking-changes/5805