static async getTemplateFile(templateName, filePath, templatesDir = DEFAULT_TEMPLATES_DIR) {
    return await readFile(path.resolve(templatesDir, templateName, filePath), 'utf8');
  }
Returns the content of a given template file.

@example
const Generator = require('@asyncapi/generator');
const content = await Generator.getTemplateFile('@asyncapi/html-template', 'partials/content.html');

@example <caption>Using a custom `templatesDir`</caption>
const Generator = require('@asyncapi/generator');
const content = await Generator.getTemplateFile('@asyncapi/html-template', 'partials/content.html', '~/my-templates');

@static
@param {String} templateName  Name of the template to generate.
@param {String} filePath      Path to the file to render. Relative to the template directory.
@param {String} [templatesDir=DEFAULT_TEMPLATES_DIR] Path to the directory where the templates are installed.
@return {Promise}