function Substitution(font) {
    Layout.call(this, font, 'gsub');
}
List all single substitutions (lookup type 1) for a given script, language, and feature.
@param {string} [script='DFLT']
@param {string} [language='dflt']
@param {string} feature - 4-character feature name ('aalt', 'salt', 'ss01'...)
@return {Array} substitutions - The list of substitutions.