[JuliaLowering] Misc. cleanup: symboliclabel, flatten_blocks

[mlechu]

Misc:

• Use _typeof_captured_variable in closure conversion
• Don't erase ex.toplevel_pure in scope resolution
• Add and use flatten_blocks to make debug output more readable.
• Remove zero-argument return from the validator and a new macro (has never been
  an accepted form in flisp lowering).

Fixups to #60481 and #61016

• Consistently use symbolic labels in break/continue instead of identifiers
• Symbolic labels don't participate in scope resolution, so don't need to be
  special-cased
• Clean up break/continue desugaring. Claude seems quite confused as to what
  forms can show up where. I can sympathize, and will hopefully have that
  written down sometime this week.
• Remove provenance-preserving @label macro---I could fix the mistakes, it's
  just certain that we need everything to work without them, and having them was
  masking some problems.
  • Naming it loop-cont/loop-exit in the normal macro and
    loop_cont/loop_exit was pretty devious

For future reference, these are the invariants we have and want to maintain:

• The expansion of any macro in syntax_macros.jl should be equal to
  expr_to_est(equivalent_old_expansion) (modulo scope layer nonsense)
• The result of Expr-macro expansion should always be lowerable by
  JuliaLowering
  • The reverse is nearly true, but requires implementation of
    scope_layer/name-mangling equivalence
• If we see a tree in desugaring, we know it passed valid_st1 and went through
  est_to_dst; no other forms are possible

Atop #61133

[Keno]

(reviewed the labeled break specific parts only)