Support variable expansion in DotEnv features

What is the problem this feature will solve?

Sometimes it's helpful to derive the value of variables from other variables, using variable expansion.

For example, consider the following .env file:

APPLICATION_PORT: 8080
BASE_URL: http://localhost:8080

The APPLICATION_PORT and BASE_URL should have the same port. For reasons external to this issue, BASE_URL cannot be split into two variables without causing confusion.

If there are external overrides to APPLICATION_PORT, they won't remain in sync with BASE_URL.

What is the feature you are proposing to solve the problem?

Instead we could write .env files like this:

APPLICATION_PORT: 8080
BASE_URL: http://localhost:${APPLICATION_PORT}

This is already supported by at least bash, and the concept of variable expansion in environmental configuration is also common in ecosystems such as Kubernetes.

What alternatives have you considered?

• Using dotenvx
• Splitting variables up into their lowest common set

该 issue 未指定文件或测试；首先定位 Node 的 DotEnv 实现及其现有测试覆盖范围。跟踪值的解析和解析结果，然后为 ${APPLICATION_PORT} 的展开以及覆盖和顺序的预期行为定义测试。完成的标准是实现所提议的展开行为，并由测试覆盖。