Include end marker information in TASTy

[bishabosha]

Compiler version

3.0.0

Summary

We should think of a way to encode end markers in TASTy, that is preferrably optional.

With #12541 we only need a single boolean to attach to a tree, so it is efficient.
The span of an end marker can then be recomputed from the tree once it is constructed.

options include:

• custom section (ok if not present)
• flag (impossible for trees with no symbol, requires TASTy version bump)
• annotation (awkward for expression trees, requires Scala version bump)