Action to handle issue dependency chains

[MarcellPerger1]

Add an action to handle issues depending on each other (e.g. Requires: #...). It should:

• Decide if we even need the label (see Action to handle issue dependency chains #48 (comment))
• Add S-blocked label when not all dependencies are met (closed).
• Remove S-blocked label when all the dependencies have been implemented/issues have been closed as completed.
• (Optional extension) generate an SVG graph of the issue dependencies.

[MarcellPerger1]

Github has now (well at some point in the last 1.5 years anyway) added blocked issues support natively so I'm unsure if the label is even required.
The SVG graph would look cool though.
Then again, the SVG graph is a complete other side project waiting in the heap of projects I want to do.

[MarcellPerger1]

Though in any case it should handle the Depends on in the issue description and add the GH-native blocked status automatically.