Refactor to split up big files

[GrahamTheCoder]

Breaking down the files should help tools - especially LLMs - deal with the codebase better and more cheaply since most don't integrate with file outlining tools yet to pull individual members.
So long as the groupings of things are sane, they will be helpful to humans at least sometimes.

Prompt

The solution has several large files in them. First find the top 10 largest .cs files. Then one at a time, assess which methods naturally group together in a semantically coherent nameable chunk, and extract it into a separate file of no more than 250 lines. Always try to have more than one method per file though. If the file is a test file, this should be simple enough, consider using folders to group the files based on their original name. For production code, e.g. the Visitor classes, they could become partial classes instead e.g. DeclaredNodeVisitor.MethodDeclaration.cs

[GrahamTheCoder]

Superseded: #1187