Enable multiple extends

[kota65535]

Feature Request

Currently, we can specify only one extends per file.
It would be helpful if we can specify multiple files in extends to combine multiple process-compose files into one.

Use Case:

Composition over inheritance

Currently, we can reuse code by inheritance using the extends keyword.
If multiple extends is available, we can do it by composition.

Depending on multiple process-compose files

Thanks for resolving #278, I could start to use this tool in monorepo.
For now, the package dependencies are so simple (server -> db) that a single extends keyword is sufficient.
We can handle more dependencies if multiple extends is allowed.

Proposed Change:

Let's make extends keyword to accept an array.

extends:
  - ../db/process-compose.yaml
  - ../lib/process-compose.yaml

Who Benefits From The Change(s)?

Users using extends keywords, especially working with monorepos.

Alternative Approaches