extensions/emmet/src/util.ts/getEmmetMode
export function getEmmetMode(language: string, excludedLanguages: string[]): string | undefined {
	if (!language || excludedLanguages.indexOf(language) > -1) {
		return;
	}
	if (/\b(typescriptreact|javascriptreact|jsx-tags)\b/.test(language)) { // treat tsx like jsx
		return 'jsx';
	}
	if (language === 'sass-indented') { // map sass-indented to sass
		return 'sass';
	}
	if (language === 'jade') {
		return 'pug';
	}
	const syntaxes = getSyntaxes();
	if (syntaxes.markup.includes(language) || syntaxes.stylesheet.includes(language)) {
		return language;
	}
	return;
}