function addMode(name, extensions, caption) {
	const filename = name.toLowerCase();
	const mode = new Mode(filename, caption, extensions);
	modesByName[filename] = mode;
	modes.push(mode);
}
Add language mode to ace editor
@param {string} name name of the mode
@param {string|Array<string>} extensions extensions of the mode
@param {string} [caption] display name of the mode