Pass Parser by exclusive reference

As described in
https://github.com/fish-shell/fish-shell/pull/9990#discussion_r1382494440,
prior to 77aeb6a2a88 (Port execution, 2023-10-08), "Parser" was
passed by mutable reference ("parser_t&"), even though operation
context was passed as "const operation_context_t &".  This worked
because C++ doesn't propagate const to pointers by default (see
https://en.cppreference.com/cpp/experimental/propagate_const).

	class operation_context_t {
		std::shared_ptr<parser_t> parser;
		...
	};

So "*ctx->parser" was a "parser_t&", not "const parser_t&".

Rust has stricter const propagation rules which means that const
operation context can't simply hand out a non-const reference to parser.

To be able to port code without changing its structure,
77aeb6a2a88 passed "Parser" by shared reference, using interior
mutability (RefCell) to modify parser fields. This is a bit ugly
(c.f. https://doc.rust-lang.org/std/cell/index.html "interior mutability
is something of a last resort") and means that some borrowing conflicts
are not found at compile time but runtime.

Pass both parser and operation context by exclusive reference, and
remove the interior mutability wrappers from parser's fields.
Since "libdata" is no longer inside a "RefCell", add a "ScopedRefCell"
around "transient_commandline".

The downside is that "ScopeGuard" use can become more intrusive
when we pass "Parser" or "OperationContext" as context (especially
when we use "zelf" since we can't shadow "self"), see
* 2930466d53b (Introduce ScopedCell and ScopedRefCell, 2025-03-15)
* 29ae571afae (Make scoped_push nicer, 2024-12-28)
Avoid this in some cases, specifically when using "ScopedCell" or
"ScopedRefCell". Since "&mut Parser" prevents the "ScopeCell"'s
"ScopeGuard" from holding a shared reference, use an "Rc" to capture
a dynamically-checked reference to the Cell. We could also use raw
pointers instead.

Change "Completer::apply_var_assignments" to return  a block ID, to
avoid the need to return a "zelf" "ScopeGuard".  In future, we could
probably untangle completer and get away with returning a "ScopeGuard"
called "ctx".

Closes #12694