fmt: fix interface fields alignment

[felipensp]

Fix #19540

🤖 Generated by Copilot at 603d820

This pull request refactors and improves the alignment logic for interface and struct fields in vlib/v/fmt. It extracts a common function for calculating the alignment, and applies it to both interface and struct declarations. It also adds a test file to verify the interface_field feature.

🤖 Generated by Copilot at 603d820

• Extract alignment calculation logic into a separate function calculate_alignment that takes fields, alignment arrays, and field types array as parameters (link, link)
• Modify interface_field function to take an additional parameter of field alignment, write spaces between field name and type, and skip writing type for anonymous structs (link)
• Declare and initialize alignment arrays and field types array for interface fields in fmt.v (link)
• Add condition to check and increment field alignment index in fmt.v (link)
• Add test files interface_field_input.vv and interface_field_expected.vv to verify alignment feature for interface fields (link, link)

[spytheman]

Excellent work.