A special “control-flow” primitive associated with the for statement.
Repeat allows for nodes to be rendered dynamically per data specified in source_expression.
That is: for a source_expression of length n, Repeat will render its
template n times, each with an embedded component context (RepeatItem)
with an index i and a pointer to that relevant datum source_expression[i]
Contains modal vec and range variants, describing whether the Repeat source
is encoded as a Vec<T> (where T is a PaxValue properties type) or as a Range<isize>