Add linter configuration to initialization options

[dbanck]

This is part of hashicorp/vscode-terraform#635

Use-cases

To enable the integration of linters within the language server, we need to accept a set of configurations upon initialization.

Proposal

• Extend the initialization properties with the linter related configuration.

terraform-ls/internal/langserver/handlers/initialize.go

Lines 59 to 71 in f246688

	properties := map[string]interface{}{
	"experimentalCapabilities.referenceCountCodeLens": false,
	"options.rootModulePaths": false,
	"options.excludeModulePaths": false,
	"options.commandPrefix": false,
	"options.ignoreDirectoryNames": false,
	"options.experimentalFeatures.validateOnSave": false,
	"options.terraformExecPath": false,
	"options.terraformExecTimeout": "",
	"options.terraformLogFilePath": false,
	"root_uri": "dir",
	"lsVersion": serverCaps.ServerInfo.Version,
	}

• Each linter will have a dedicated section in the configuration
• The first linter we plan to support is tflint, new options might look like this:
  • options.linters.tflint.binaryPath
  • options.linters.tflint.binaryFlags
  • options.linters.tflint.lintOnSave
• Validate the linter configuration and check if the passed binary is accessible and executable
• If validation fails, fail silently and disable any internal linting code
• If validation succeeds, the linters can be consumed on safe or via a command (separate issues)

[radeksimko]

Just noting here that the settings here should ideally follow the principles outlined as part of #1010

i.e. on the LS side this could look like this

{
  "linters": {
    "tflint": {
      "binaryPath": "",
      "binaryFlags": [""],
      "lintOnSave": true
    }
  }
}

and VS Code may expose the same options as

{
  "linters.tflint.binaryPath": "",
  "linters.tflint.binaryFlags": [""],
  "linters.tflint.lintOnSave": true