fix: apply FuncMap parsers to pointer-to-custom-type fields

[Solaris-star]

Summary

When a field is a pointer to a custom struct type and has its own env tag, doParseField always recursed into nested fields and never applied FuncMap parsers / setField for that pointer field.

That made this fail:

type foo struct{ name string }
type config struct {
  Foo *foo `env:"BLAH_CUSTOM"`
}
ParseWithOptions(&cfg, Options{FuncMap: map[reflect.Type]ParserFunc{
  reflect.TypeOf(foo{}): func(v string) (any, error) { return foo{name: v}, nil },
}})

Foo kept its pre-parse value instead of being rebuilt from the env string.

Fix

1. In doParseField, only recurse into *Struct when the field has no own env key (and field-name defaulting is off). Own keys fall through to setField.
2. In set, allocate a nil pointer target before writing a custom-parser value (so NilFoo *foo works too).

Test plan

• TestCustomParserPointerToCustomType (includes nested Other *bar + nil pointer target)
• full go test . (with ambient PORT unset)

Fixes #385