Add strategy for extension and skin of MediaWiki

**Is your feature request related to a problem? Please describe.**
**Describe the solution you'd like**
[MediaWiki] has its extension and skin system. Each extension or skin exists on its repository and is released separated by the MediaWiki core.

- Anyone can create and maintain an extension or skin.
- The version number is recorded at `skin.json` or `extension.json` file.

I want to use Release Please for it.

**Describe alternatives you've considered**
I have created a fork and am using it.
**Additional context**
There is no standardized CHANGELOG.md convention. In further, it seems the developers of MediaWiki ecosystem commonly do not write anything.

- Extension:Notifications (previously "Echo") - https://github.com/wikimedia/mediawiki-extensions-Echo/blob/master/RELEASE_NOTES
- Extension:Wikibase - https://github.com/wikimedia/mediawiki-extensions-Wikibase/blob/master/RELEASE-NOTES-1.44
- Vector, DiscussionTools, GrowthExperiments, etc. have no notes
https://github.com/wikimedia/mediawiki-skins-Vector
https://github.com/wikimedia/mediawiki-extensions-DiscussionTools
https://github.com/wikimedia/mediawiki-extensions-GrowthExperiments
https://github.com/wikimedia/mediawiki-extensions-Chart

[mediawiki]: https://www.mediawiki.org