Generate JSON Schema and Markdown doc for .sourcekit-lsp/config.json

[kateinoigakukun]

Providing a JSON schema for the configuration file should improve the developer experience thanks to better auto-completion and diagnostics support provided by some editors.

Additionally, we have been manually maintaining the configuration file format documentation in Documentation/Configuration File.md, but it's easy for the documentation to get out of sync with the actual schema.

This change introduces a new tool, ConfigSchemaGen, that generates a JSON schema and a Markdown document for the configuration file based on the Swift type definitions in the SKOptions module by using swift-syntax.

Plan for vscode-swift integration

VSCode exposes a way to feed a JSON Schema definition for an opening file based on file name:
https://code.visualstudio.com/api/references/contribution-points#contributes.jsonValidation

Contribute a validation schema for a specific type of json file. The url value can be either a local path to a schema file included in the extension or a > remote server URL such as a json schema store.

{
  "contributes": {
    "jsonValidation": [
      {
        "fileMatch": ".jshintrc",
        "url": "https://json.schemastore.org/jshintrc"
      }
    ]
  }
}

Since the schema URL can be a local path and the configuration field can be modified dynamically by extension, we can specify a schema file shipped in the selected Swift toolchain. Or to simplify packaging flow, we can embed the schema file within the sourcekit-lsp binary and provide a CLI interface to print it, then vscode-swift can write it out in its extension storage and feed the file to VSCode.

[kateinoigakukun]

@swift-ci test