Being stuck at home, bored, desperate and coming up with a lot of weird ideas, a friend asks you to develop a tool to measure his ocular skills. One of those [Monoyer charts](https://en.wikipedia.org/wiki/Monoyer_chart) that ophthalmologists use.
Write the function `generateLetters` which creates 120 `div` elements, each containing a letter randomly picked through the **uppercase** alphabet, and whose style properties have to be increased:
- each letter's `font-size` has to grow from `11` to `130` pixels.
-`font-weight` has to be `300` for the first third of the letters, `400` for the second third, and `600` for the last third.