Add Prettier module augmentation for custom options

Problem

The plugin's custom options (xmlWhitespaceSensitivity, xmlSelfClosingSpace, xmlSortAttributesByKey, xmlQuoteAttributes) are not typed on Prettier's Options interface. Consumers get no autocomplete or type narrowing for these options in TypeScript Prettier configs.

Other Prettier plugins (e.g. prettier-plugin-sort-json) include a declare module 'prettier' augmentation:

declare module 'prettier' {
  interface Options {
    xmlWhitespaceSensitivity?: 'strict' | 'preserve' | 'ignore' | undefined;
    xmlSelfClosingSpace?: boolean | undefined;
    // ...other options
  }
}

Expected behavior

Importing the plugin should augment Prettier's Options interface with the plugin's custom options, providing autocomplete and type safety in TypeScript configs.

Locate the plugin's package entry and any existing type declaration files, then inspect how its custom options are exposed to Prettier. Add the module augmentation for all four options and verify that TypeScript configs receive autocomplete and type checking for them.