support complex map types, where the map value can be a struct

The proposal is to support complex map types so that we could do the following

```
type Test struct {
Str string `env:"DAT_STR"`
Num int `env:"DAT_NUM"`
}
type ComplexConfig struct {
Bar map[string]Test `envPrefix:"BAR_"`
}

t.Setenv("BAR_KEY1_DAT_STR", "b1t")
t.Setenv("BAR_KEY1_DAT_NUM", "201")
```

This is similar to https://github.com/caarlos0/env/issues/298 but for maps

The key can be any supported value (that are already present in either `opts.FuncMap` or the default parser list in `defaultBuiltInParsers`).

I will open a PR for this Issue.

Start by tracing the existing map handling and the parser paths named in the issue: opts.FuncMap and defaultBuiltInParsers. Implement support for map values that are structs, including envPrefix-based nested fields and supported key types, then verify the provided ComplexConfig example and related parsing behavior.

Written by the indexing model from the issue text.