function registerContextChecker(checkId) {
    var check = this.contextChecks[(checkId + "Check")];
    return this.tokenizer.registerContextChecker(
        checkId, check.startCheck, check.endCheck
    );
}
Apply GSUB features
@param {Array} tagsList a list of features tags
@param {string} script a script tag
@param {Font} font opentype font instance