function testCurrentChar() {
                    const char = parserInput.currentChar();
                    if (typeof tok === 'string') {
                        return char === tok;
                    } else {
                        return tok.test(char);
                    }
                }
Used for custom properties, at-rules, and variables (as fallback)
Parses almost anything inside of {} [] () "" blocks
until it reaches outer-most tokens.

First, it will try to parse comments and entities to reach
the end. This is mostly like the Expression parser except no
math is allowed.

@param {RexExp} untilTokens - Characters to stop parsing at