Add documented struct field spacing option

[joshka]

Fixes #6925.

Summary

Add unstable rustfmt options for inserting blank lines around documented struct field sections:

• documented_struct_field_blank_lines = "Preserve" | "Always" | "Threshold"
• documented_struct_field_blank_lines_threshold = <usize>

This follows existing rustfmt config patterns better than overloading a single numeric option with a sentinel value.

Testing

• cargo test --lib test_dump_default_config -- --nocapture
• cargo test --lib test_dump_style_edition_2024_config -- --nocapture
• cargo test --lib verify_config_test_names -- --nocapture
• cargo test --lib configuration_snippet_tests -- --nocapture
• cargo test --lib system_tests -- --nocapture

Implementation notes

• The threshold is evaluated against the total number of fields in the struct, not the length of a contiguous documented run.
• Once a struct qualifies, spacing is applied per contiguous documented section within that struct.
• The implementation reuses ListItem::new_lines in the vertical field formatter instead of introducing a second spacing path.

Open points / context

• While implementing this, the original numeric-only shape turned out to be ambiguous around 0 and 1, especially if 0 was treated as either Preserve or Always. The enum mode avoids that ambiguity.
• The current tests cover the mode differences and threshold boundaries. They do not add dedicated interaction coverage for cases like #[rustfmt::skip] or intervening non-doc comments inside documented sections; those continue to follow the existing contiguous-section logic in vertical.rs.

AI disclosure

This PR was developed with assistance from Codex using GPT-5.5.

[ytmimi]

Closing since we're not moving forward with the feature request at this time.