Variable definitionListRuleConst

Block rule for Wikidot definition lists (: key : value).

Parsing strategy:

1. Verify the first token is a line-start COLON.
2. Repeatedly call parseDefinitionItem() to collect entries.
3. Stop when the current token is no longer a line-start COLON (i.e. the definition list block has ended).
4. Convert internal items into the AST DefinitionListItem format.
5. Emit a single definition-list element.