[JuliaLowering] Add `soft_scope` kwarg to `resolve_scopes`
Add `soft_scope::Union{Nothing,Bool}=nothing` keyword argument
to `resolve_scopes` so callers can explicitly control soft scope
behavior without inserting `K"softscope"` marker nodes into the
AST. When `nothing` (the default), the existing marker-based
detection is used.
This is useful for tools like JETLS that lower code from
different contexts (files vs notebooks). Notebook cells use
REPL-like soft scope semantics, and with this kwarg, the
language server can pass `soft_scope=true` directly rather
than manipulating the AST to insert marker nodes.
