Disallow Metadata to be Placed on Nested Module Syntax

[InsertCreativityHere]

For 3.8, we added support for nested module syntax:

module Foo::Bar {...}

This is great, but it introduces some unfortunate ambiguity with how metadata applies to this syntax.
For example, if you write:

["something"]
module Foo::Bar {}

I think there are really 3 separate ways to interpret this:

1. Metadata applies to the innermost module (Bar)
2. Metadata applies to the outermost module (Foo)
3. Metadata applies to the entire construct (Foo::Bar)

I think all 3 of these are valid intuitive beliefs. So no matter which we pick, some users will be surprised, and all users will be uncertain.
So, I think we should pick the secret option 4: Disallow metadata to be placed on nested-module definitions.

---

This isn't even my idea. Option 4 is what Rust and C++ do. For example, compiling this causes a syntax error:

namespace [[deprecated]] Foo::Bar::Baz
{
    struct Hello { int i; };
}

/tmp/j2zpwYlh9a/main.cpp:4:1: error: a nested namespace definition cannot have attributes
    4 | namespace [[deprecated]] Foo::Bar::Baz
      | ^~~~~~~~~

But it's totally okay with attributes on 'normal' modules:

namespace [[deprecated]] Baz
{
    struct Hello { int i; };
}

/tmp/f2wtwFCLHt/main.cpp: In function 'int main()':
/tmp/f2wtwFCLHt/main.cpp:13:16: warning: 'Baz' is deprecated [-Wdeprecated-declarations]
   13 |     Baz::Hello there {5};

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Since nested-module syntax is new for 3.8, we aren't loosing anything by not supporting this.
Of course, we can revisit this in the future if supporting metadata with nested-module syntax becomes a requested feature.