Add a language to describe CHANGELOG.md files

[alexanderpann]

It is based on jetbrains.mps.lang.text and not de.slisson.mps.richtext and uses com.dslfoundry.plaintextflow for generating the Markdown file. There is some additional documentation in the inspector of the Changelog concept:

The generated file is currently copied to the root directory with the Gradle task copyReadme which should be improved. The language supports the change headers formats version + date and month + year:

First format:

Second format:

[alexanderpann]

auto create bullet-lists when entering -

This is already supported by entering "*" instead of "-".
I've implemented the sorting order check but not the automatic replacement of the backticks. I am not sure how you correctly extend the language jetbrains.mps.lang.text. I'd love to see something like a floating palette or the context actions tool to host such formatting actions.