Document Symbols are used to provide an outline of a document.
By default, no document symbols are provided.
To define that a rule denotes a document symbol, one can override the property SymbolKind of the class Rule. Because LSP restricts the possible valuies of a document symbol kind, this is implemented as an enumeration.
Making a rule representing a document symbol also registers that symbol kind for completions and activates a folding range by default.
Symbols can be tagged, overriding the method SymbolTag.