feat: Null Value Support

[getBoolean]

Closes #25

Annotation options Envied.allowOptionalFields and EnviedField.optional have been added to allow nullable fields. Optional fields are disabled by default. Below is an excerpt from the readme.

Optional Environment Variables

Enable allowOptionalFields to allow nullable types. When a default
value is not provided and the type is nullable, the generator will
assign the value to null instead of throwing an exception.

By default, optional fields are not enabled because it could be
confusing while debugging. If a field is nullable and a default
value is not provided, it will not throw an exception if it is
missing an environment variable.

For example, this could be useful if you are using an analytics service
for an open-source app, but you don't want to require users or contributors
to provide an API key if they build the app themselves.

@Envied(allowOptionalFields: true)
abstract class Env {
    @EnviedField()
    static const String? optionalServiceApiKey = _Env.optionalServiceApiKey;
}

Optional fields can also be enabled on a per-field basis by setting

@EnviedField(optional: true)

[techouse]

Good work. I've added some comments. 🤓

[getBoolean]

Thanks for the feedback!

Please check that #66 (comment) and #66 (comment) was resolved correctly.

[techouse]

Please check that #66 (comment) and #66 (comment) was resolved correctly.

Looks like it. I'll make a proper review later in the evening or tomorrow.

[techouse]

Added some more comments.

[techouse]

Looks good. 👍🏻 I'll leave the PR open for a few more days in case you decide to add more stuff to it.

@petercinibulk care to take a look?