Add support for changing the order of attributes/blocks

[skyzyx]

Looking to be able to enable support for auto-updating a user's ~/.terraformrc file, assuming it exists.

The official docs state:

As a convenience for provider development, Terraform supports a special additional block dev_overrides in provider_installation blocks. The contents of this block effectively override all of the other configured installation methods, so a block of this type must always appear first in the sequence: [code example]

I see that there is support for append, but what about adding support for prepend?

[minamijoyo]

Thank you for your proposal!
While supporting the .terraformrc is technically impossible, as mentioned in #84, I think inserting a node at a given position is still a valid feature request.
However, the current problem is the lack of APIs of the underlying hclwrite library. hashicorp/hcl#589 So, I'll leave this open.