Allow `JSON` for `TF_LOG_PROVIDER_{provider}` configuration

### terraform-plugin-log version

```
v0.7.0
```

### Relevant provider source code

n/a

### Terraform Configuration Files

n/a

### Expected Behavior

From the [documentation](https://developer.hashicorp.com/terraform/plugin/log/managing) I would expect that e.g. setting `TF_LOG_PROVIDER_{provider}=JSON` would result in logs in JSON with `TRACE` log level.

### Actual Behavior

Using `TF_LOG_PROVIDER_{provider}=JSON` doesn't configure logging at all.

Setting `TF_LOG=JSON` works as expected though. And I cannot combine them since `TF_LOG` overrides the others.

### Steps to Reproduce

Just set `TF_LOG_PROVIDER_{provider}=JSON` for any *provider* when running Terraform.
I'm using it when running provider acceptance tests.

### References

Start by tracing how TF_LOG and TF_LOG_PROVIDER_{provider} environment variables are parsed, then reproduce the issue with a provider-specific value of JSON. Done means the provider-specific setting enables TRACE-level JSON logging without being ignored or requiring the global TF_LOG variable.

Written by the indexing model from the issue text.