fmt: keep anon struct decl fields in interfaces

[Delta456]

Closes #19459

🤖 Generated by Copilot at d7fe90f

Add support for anonymous structs inside interfaces in the V language. Modify vlib/v/fmt/fmt.v to format such declarations correctly and add a test file vlib/v/fmt/tests/interface_anon_struct_decl_keep.vv.

🤖 Generated by Copilot at d7fe90f

• Add support for anonymous structs inside interfaces (link, link)
  • Modify write_struct_decl method of Fmt struct in vlib/v/fmt/fmt.v to check for anonymous struct fields and write them using write_anon_struct_field_decl method (link)
  • Add write_anon_struct_field_decl method to Fmt struct in vlib/v/fmt/fmt.v to write the field name and the struct declaration for anonymous struct fields (link)
  • Add test file vlib/v/fmt/tests/interface_anon_struct_decl_keep.vv to define an interface with an anonymous struct field and check that the formatter keeps the struct declaration as it is (link)

[spytheman]

(rebased over master)