Assign the name in Unit column to Unit Display Name

[mairas]

Improves multi-unit symbol handling by properly assigning custom unit names from CSV data to KiCad symbol unit display names, enabling better organization of complex components with named functional units.

Changes

• Enhanced unit name handling: Modified symbol_to_csv_rows() to extract custom unit names from KiCad symbols instead of generating generic numbered names
• Added unit_name support: Updated rows_to_symbol() to write custom unit names to KiCad symbol definitions for multi-unit components
• Improved CSV output: Unit column now shows actual unit names (like "1") rather than prefixed identifiers (like "my_part_1")
• Updated test expectations: Modified unit tests to reflect the cleaner unit name output format