feat(semantic-release): don't build changelog on prerelease branches

By default, `semantic-release` build changelog even for prerelease
branches which looks like duplicated entries to users.

Use a workaround to avoid changelog generation for prerelease branches
by building the configuration in several steps instead of a direct
`module.exports` at the top.

There are no effect on this project since the `branches` configuration
attribute only contains `stable` but can be used for other project
with the only change to declare prerelease `branches`.

* release.config.js: rework the configuration file to conditionnally
  execute `@semantic-release/changelog` and add its assets to the git
  plugin.