[two_dimensional_scrollables] Refactor Spans for common use (#6550)
This relocates most of table_span.dart to ../common/span.dart, renaming the classes to basic Spans instead of TableSpans.
This is because since a Span can represent a row or a column, the new base Span class will be used to build TreeRows in the upcoming TreeView.
In table_span.dart, the old classes are now typedefs. This is because (IMO) the Table and Tree APIs are better to work with when the naming conventions reflect what you are building. The Table and Tree uses of Span could also deviate in the future and it felt prudent to leave room for future changes that may be specific to only one or the other.
Otherwise a bit of tidying to get ready for TreeView!
Part of https://github.com/flutter/flutter/issues/42332
[two_dimensional_scrollables] Fix TreeView null dereference during paint (#9103)
Collapsing a node when there were other nodes offscreen was causing an unexpected null dereference during painting. This PR fixes the bug and adds a test.
The bug was caused by erroneous computation of the max vertical scroll extent. Previously, the code computed this considering only scroll extent in the trailing (down) direction; it may also be the case that there is a larger scroll extent in the leading (up) direction.
The miscalculation resulted in subsequent error computing the first visible row as a row that is actually offscreen, and thus does not have a render box. The row render box is asserted to be non-null during painting.
Fixes https://github.com/flutter/flutter/issues/149182 and https://github.com/flutter/flutter/issues/164981
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