Add unscoped variables support

[CheerlessCloud]

I want add ability to map raw variables without MODULE__ prefix for simplify migration.

In this case config definition will be similar to this:

@UnscopedConfig()
export class LegacyConfig {
  @Env('LEGACY_VARIABLE')
  @String()
  readonly fieldOne: string;
}

For details see changes in README and e2e tests.

[feeedback]

Или "^9.1.4" ?

[Goodluckhf]

there will be different behaviour with @UnscopedConfig and usuall @Config. So after migration could be implicit consequences

[CheerlessCloud]

Yep, but without excludeExtraneousValues all envs will be automatically exposed to class instance which is more terrible than the inconsistent behavior. Moreover, I do not understand why this option is not enabled in the @Config mode. And, I hope, @UnscopedConfig is more for migration cases than for everyday use, because implementation is one big workaround.

I think we should describe excludeExtraneousValues behavior in readme of @UnscopedConfig and enable it for @Config in next major version.

P.S. But in general, you gave me a strange idea how to reduce workaroundness of this solution, I'll try to implement it.