[Feature Request] configure blank lines between related items

related to: https://github.com/rust-lang/rustfmt/issues/3382

I'd like to request an option to specify the amount of blank lines between related items.

Related items either share the same concrete Type:

• struct A
• enum A
• impl A
• impl<T> A
• impl Trait for A

Or the same Trait type:

• trait T
• impl T for A

Formatting rule

• Between items in the same group: N blank lines. (default to 0 or 1)
• Between different groups: exactly M blank lines (typically 1).

Example normalization (N = 0, M = 1):

struct A;
impl A {
  //...
}

struct B;
impl B {
  //...
};

Example configuration could be

top_level_items_related_blank_lines = 0
top_level_items_unrelated_blank_lines = 1

Inferring macros would be out of scope.

Start by reading the related rustfmt issue #3382 and the requested grouping examples. Define how related top-level items are identified, how the two blank-line configuration options interact, and what defaults apply. Done means rustfmt consistently separates related and unrelated items as specified, without requiring macro inference.