…GetLinesCore.

Going to start this in draft mode and fire off a test insertion to verify this improves allocations as the typing scenario in speedometer scrolling test shows this as nearly 10% of allocations.

The general idea here is that ChangedText doesn't need to keep track of line information as the SourceText that it wraps has that information. The complexity that was in ChangedText around newline splitting now needs to sit in both CompositeText and SubText as they need to understand how to expose their line collections.

…over all possible source texts generatable from that sequence

2) Move the two added TextLineCollection derivations to be private.

… it more clear

…handling in two different locations, just handle it in a single location up front.

…ome data in the empty SubText.UnderlyingSpan case.