Let empty files be empty files

[notpeter]

No longer add a "final newline" to empty files.
Empty files shouldn't need need trailing newlines.

• Fixes: rustfmt fails on an empty file; --check's diff doesn't result in properly formatted file #5364

Since this is a breaking change, I've done my best to feature gate this behind StyleEdition::Edition2027. Please let me know if this should be handled differently.

New tests:

• append_newline_adds_newlines_before_2027
• append_newline_leaves_empty_files_empty_in_2027
• newline_file (existing 2024 behavior; excess newline is preserved)
• reorder_modules_2027 (2027 behavior test extracted from reorder_modules)

The empty file mod.rs test fixtures for the reorder_modules test were previously being re-used for multiple editions (2015, 2024, 2027), but since 2027 now requires those files be actually empty instead of \n, I moved {enabled,disabled_style_edition_2027.rs} from reorder_modules/ to a new reorder_modules_2027/ and mirrored the mod.rs test fixture structure.

The existing empty_file.rs test (empty file becomes newline) remains untouched. (config not specified; so 2015).

Update1: Fixed reorder_modules
Update2: 2027 tests pass on nightly, fail on stable. Unsure of the proper fix.
Update3: Renamed empty_file_style_edition_2024 to newline_file because it's not actually an empty file.
Update4: Implemented ytmimi's suggestion and fixed tests.

[ytmimi]

Please investigate test failures. Also see my suggestion for simplifying the conditional logic.

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