Natural language doesn't work with percentages

Describe the issue Using the words 'of', 'off' and 'on' breaks expression evaluation and doesn't produce any output.

To Reproduce Steps to reproduce the behavior:

Type in an example with percentages from the docs:
```
10% of 100
10% on 100
10% off 100
```

100 + 5% After discount: 19.99 - 20%


2. Observe no output for expressions with the words 'of', 'off' and 'on'.

**Screenshots**
![image](https://user-images.githubusercontent.com/8556514/213908766-56ff093a-d66a-4601-8376-e70effe71150.png)

**Device**
 - OS: Windows 10
 - Version 2.0.0-rc.3

parsify-dev / desktop

Natural language doesn't work with percentages #83