feat: support multiple branches and distribution channels

- Allow to configure multiple branches to release from
- Allow to define a distribution channel associated with each branch
- Manage the availability on distribution channels based on git merges
- Support regular releases, maintenance releases and pre-releases
- Add the `addChannel` plugin step to make an existing release available on a different distribution channel

BREAKING CHANGE: the `branch` option has been removed in favor of `branches`

The new `branches` option expect either an Array or a single branch definition. To migrate your configuration:
- If you want to publish package from multiple branches, please the configuration documentation
- If you use the default configuration and want to publish only from `master`: nothing to change
- If you use the `branch` configuration and want to publish only from one branch: replace `branch` by `branches` (`"branch": "my-release-branch"` => `"branches": "my-release-branch"`)

test/definitions/plugins.test.js

@@ -32,6 +32,16 @@ test('The "publish" plugin output, if defined, must be an object', t => {
   t.true(plugins.publish.outputValidator(''));
 });
 
+test('The "addChannel" plugin output, if defined, must be an object', t => {
+  t.false(plugins.addChannel.outputValidator(1));
+  t.false(plugins.addChannel.outputValidator('string'));
+
+  t.true(plugins.addChannel.outputValidator({}));
+  t.true(plugins.addChannel.outputValidator());
+  t.true(plugins.addChannel.outputValidator(null));
+  t.true(plugins.addChannel.outputValidator(''));
+});
+
 test('The "generateNotes" plugins output are concatenated with separator and sensitive data is hidden', t => {
   const env = {MY_TOKEN: 'secret token'};
   t.is(plugins.generateNotes.postprocess(['note 1', 'note 2'], {env}), `note 1${RELEASE_NOTES_SEPARATOR}note 2`);