support @synopsis in Rascal summary creation for hover help

[jurgenvinju]

Describe the bug

@synopsis and @description were added as tutor-supported tags next to the existing @doc{.Synopsis ... .Description ...}

• It would be great if the type-checker would extract synopsis from functions that are tagged @synopsis as well.
• Tutor supports either @doc or @synopsis but not both at the same time for the same declaration
• Eventually @doc might dissappear
• Currently @doc is used to supply the information for hover help in the Eclipse IDE and the VScode IDE.