add terraform_json_syntax rule

[bendrucker]

Adds a new rule to enforce Terraform's official JSON syntax, which requires a root object rather than an array. HCL supports both syntaxes:

https://github.com/hashicorp/hcl/blob/main/json/spec.md

The HCL syntax-agnostic information model defines a body as an abstract container for attribute definitions and child blocks. A body is represented in JSON as either a single JSON object or a JSON array of objects.

Terraform's documentation specifies that JSON configurations should use a root object with keys for each block type (resource, variable, output, etc.). It inherits HCL's support for arrays and flattening them, but it never documents this.

Changes

• Adds terraform_json_syntax rule that detects array syntax at the root of .tf.json files.
• Uses encoding/json to unmarshal and check the root type structure.
• Emits a warning when array syntax is detected, pointing users to the official Terraform JSON syntax documentation.

Testing

• Added unit tests covering valid object syntax, invalid array syntax, non-JSON files, and malformed JSON.

References

• TFLint errors on JSON syntax with root array of blocks tflint#2279
• See fix: support array-based JSON syntax in WalkExpressions tflint-plugin-sdk#422, which allows TFLint to parse expressions with this syntax, where currently it errors if you try to use WalkExpressions

[bendrucker]

Trimmed the error and added autofix. It's a lot of code compared to just handling detection, but it's also a major annoyance/confusion to eliminate for users.