Add 0.19.0 changelog entry for mkPartitionModule and lib refactoring

Adds changelog entry for 0.19.0 covering:

• lib.mkPartitionModule for composable partition setup (!57)
• Library refactoring into per-function files under nix/lib/
• flake.lib declared as mergeable option

This was missing from the !57 merge. Once merged, tag 0.19.0.

/cc @nastypants @smartypants @locallycompact