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:

[psuzzi]

The Changelog contains [0..1] header:Text and [1..n] releases:Release.
The Release holds

• [1] header IReleaseHeader, that concretely can be MonthYearReleaseHeader or VersionDateReleaseHeader (the latter allows for SemanticVersion)
• [1..n] sections: ChangeSection, each of which is a [1..n] collection of Line with elements: TextElement

The generator is well-thought for generating .md files (for now), and is extensible to other formats.

The editors provide the basic editing functionalities, mostly through intentions.

This initial submission for the language looks good to me.

For the future, I would consider editor improvements, such as warning when Release entries are not in reverse-chronological order and auto-transform code when entering ` ` (backticks) or auto create bullet-lists when entering - (but this would make the language more specific - hence a discussion will be needed)

[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.