Fix IfStmt single-statement branch accessors

[WindOctober (WindOctober)]

Fixes #287

Select if branch bodies in source order across both BlockExpr and Stmt
nodes, so else_branch_stmt() does not return the then branch.

Adds a regression test for distinct single-statement then and else branches.

Tested with:

cargo test -p oq3_syntax --test if_stmt_branches

[CLAassistant]

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