Describe the bug
If a new tailscale_acl resource is getting created, it will delete and override an already existing ACL. This is the worst possible behaviour imaginable for a resource and can mess people up big time, especially since the Tailscale WebUI doesn't offer revision control of the ACL to restore previous ACLs. I would even suggest to pull this resource from circulation in an emergency release until this issue is fixed, since it is dangerous and doesn't heed the most basic concepts of a terraform resource one would expect. A "create" action should never ever override existing items.

So 2 things: If this resource continues to override the ACL wholesale then it should only do so after a successful (so far apparently unimplemented) import of the resource, and otherwise error. However, such a resource will be pretty much unusable for all but the most simple use cases, since usually all the Subnet Routers will live in different VPCs of different projects, and will be implemented individually as part of these projects. Resources aren't separated by provider in Terraform, but by organisational structures within a company.

Therefore the proper fix would be to make this resource merge its policy items into an already existing ACL.

To Reproduce
Steps to reproduce the behaviour:

1. Create an ACL in tailscale WebUI manually
2. Create a different tailscale_acl via terraform, and apply it against the same tailnet.
3. The manually created ACL will get overridden

Expected behaviour
Never ever delete existing infrastructure/config without a lot of warning!

Desktop (please complete the following information):

• OS: Linux
• Terraform Version 1.2.9
• Provider Version 0.13.5