Support non‑consecutive indices when parsing slice environment variables

# Brief Description

When faced with slice element environment variables with discontinuous names (e.g., `PREFIX_0`, `PREFIX_2`, `PREFIX_3`), it is impossible to obtain all variable values.

# Current Behavior

Currently, with `PREFIX_0`, `PREFIX_2`, `PREFIX_3`, only `PREFIX_0` is retrieved.

# Expected Behavior

The slice variable should retrieve PREFIX_0, PREFIX_2, and PREFIX_3, and preserve a placeholder (e.g., a zero value) at the missing index 1.

# Steps to Reproduce

```go
type Test struct {
Str string `env:"STR"`
Num int `env:"NUM"`
}
type Config struct {
Foo []Test `envPrefix:"FOO"`
}

os.Setenv("FOO_0_STR", "a")
os.Setenv("FOO_0_NUM", "1")
os.Setenv("FOO_1_STR", "b")
os.Setenv("FOO_1_NUM", "2")
os.Setenv("FOO_3_STR", "d")
os.Setenv("FOO_3_NUM", "4")

var cfg Config
if err := Parse(&cfg); err != nil {
fmt.Println(err)
}
fmt.Printf("%+v\n", cfg)
// Expect: {Foo:[{Str:a Num:1} {Str:b Num:2} {} {Str:d Num:4}]}
// Got: {Foo:[{Str:a Num:1} {Str:b Num:2}}
```

# Root Cause Analysis

`doParseSlice()` does not support discontinuous environment variable index names; it assumes indices are consecutive. For environment variables `PREFIX_0`, `PREFIX_2`, `PREFIX_3`, it finds that `PREFIX_1` does not exist and stops counting. The parser should continue parsing up to the highest index present in the environment variables, preserving gaps between indexes.

https://github.com/caarlos0/env/blob/6c3de6f047c2bab82da5029c23a36e461d8499c9/env.go#L450-L461

Start by inspecting doParseSlice() in env.go around lines 450-461 and reproduce the discontinuous-index case from the issue. Done means parsing continues through the highest present index, retains missing positions as zero-value elements, and produces the expected Foo slice with entries at indexes 0, 2, and 3.

Written by the indexing model from the issue text.