Add JSON configuration file support

[rorychatterton]

closes #2389

This change adds support for JSON-formatted TFLint configuration files (.tflint.json) in addition to the existing HCL format (.tflint.hcl).

Changes:

• Update config loading to detect file extension and parse JSON/HCL accordingly
• Add support for .tflint.json in default file discovery
• Maintain HCL preference over JSON files
• Add unit tests for JSON config loading scenarios
• Add integration tests for JSON config functionality and precedence
• Update user guide documentation with JSON format examples

The implementation uses the existing hclparse.ParseJSON() from the HCL library, following the same JSON structure conventions as Terraform's .tf.json files. Existing functionality remains unchanged.

Other:

• Unit Tests Pass
• Integration Tests Pass in CI
• Confirm Source Map - As in the other thread, need to confirm that the sources data structure supports JSON, or whether we need to create an intermediary representation as HCL
• Update Bundled Plugin to use SDK 0.23.0

For full transparency, I used Claude Code to help generate this commit, but am responsible for reviews etc.

[rorychatterton]

This PR has been updated to include a compatibility shim for plugins using SDK versions < 0.23.0.

The Problem

Plugins using SDK < 0.23.0 cannot parse JSON config files with .json extensions - they only recognize .tf.json as valid JSON. This causes errors when rules with custom parameters try to parse the config.

The Solution

Added a compatibility shim that:

• Detects plugin SDK versions during initialization
• For plugins with SDK < 0.23.0, the shim renames the json file to tf.json and reparses the configs to ensure the paths are correct.
• Logs warnings to inform users about compatibility mode
• Ensures all third-party plugins work regardless of SDK version

Current Test Status

The integration test is failing because the bundled terraform ruleset bypasses the init version checks that apply to third-party plugins. Rather than adding special-case handling for the bundled ruleset, the simpler solution is to wait for this PR to be merged and SDK v0.23.0 to be published.

Once the SDK is updated, the bundled plugin will natively support .json extensions and all tests will pass. The compatibility shim will continue to ensure backward compatibility for any third-party plugins still using older SDK versions.

[rorychatterton]

Sorry about the commit noise. I use Graphite to manage my CI in the backend and it gets a little pushy.

This should be ready to review now. In summary:

Changes by Component

Plugin Version Checking (plugin/plugin_version.go)

• Extracted all version validation logic from cmd/inspect.go into
  centralised functions
• Created CheckTFLintVersionConstraints() and
  CheckSDKVersionSatisfiesConstraints()
• Added version constants: DefaultSDKVersionConstraints (>=0.16.0),
  JSONConfigSDKVersionConstraints (>=0.23.0)

Config Loading (tflint/config.go, tflint/config_test.go)

• Modified LoadConfig() to detect JSON files by extension and parse
  accordingly
• Added isJSONConfig field and IsJSONConfig() method to track config
  type
• Updated tests to cover JSON loading scenarios

Inspection Flow (cmd/inspect.go)

• Removed inline version checking code, now calls centralized functions
• Split checks: TFLint constraints in launchPlugins() before
  ApplyGlobalConfig, SDK checks in ValidatePluginVersions() after

Language Server (langserver/handler.go)

• Updated to use centralized version checking functions
• Now properly detects JSON configs via cfg.IsJSONConfig() and enforces
  appropriate SDK requirements

Testing

• Added plugin/plugin_version_test.go with SDK version validation tests
• Added integration tests in integrationtest/cli/ for JSON config loading
  and HCL/JSON precedence

[bendrucker]

Nice work here! Couple minor code readability comments, if you have time, it would be helpful. Otherwise I can make those edits.

[bendrucker]

A switch expression with case expressions is easier to read than a bare switch:

Suggested change

	switch {
	case strings.HasSuffix(strings.ToLower(file.Name()), ".json"):
	f, diags = parser.ParseJSON(src, file.Name())
	default:
	f, diags = parser.ParseHCL(src, file.Name())
	}
	switch filepath.Ext(file.Name()) {
	case ".json":
	f, diags = parser.ParseJSON(src, file.Name())
	default:
	f, diags = parser.ParseHCL(src, file.Name())
	}

You'll need to import filepath too

[bendrucker]

Declare the empty config earlier so you can just do config.isJSONConfig = true in the switch statement rather than repeating the extension check.

[bendrucker]

Alternatively, you could just store the config path on the config object, unexported. Or expect it as the first/only key in sources. And then call return filepath.Ext(filename) == ".json" here

[rorychatterton]

I'm travelling without my work computer til the 5th of November. Happy to make the changes on my return, else feel free to make them :).

[wata727]

👍

Once you've addressed Ben's comments, we can merge this PR.

[wata727]

Is there a reason why you changed the package name?