Rename PreorderVisitor to SourceOrderVisitor

[MichaReiser]

Summary

There has been some confusion about the order in which PreoderVisitor visits the AST nodes because it is poorly named (I'm allowed to say this, I wrote it ;))

This PR renames the PreorderVisitor to SourceOrderVisitor and hints users in the documentation to to Visitor that does evaluation order visiting.

Test Plan

cargo test

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[AlexWaygood]

Nice, this is much clearer IMO

[carljm]

Looks great!