Fix cursor navigation and backspace functionality CodeTool

[idebenone]

Problem:

Users were unable to delete text with backspace and navigate with prefilled data.

Solution:

Update textarea.textContent to textarea.value while getting and setting code data.

closes #67

[neSpecc]

Have you discovered why this problem appears only when tool rendered with saved data? If you add the new block, it can't be reproduced

[idebenone]

Have you discovered why this problem appears only when tool rendered with saved data? If you add the new block, it can't be reproduced

The first code block is rendered with saved data and you can see it between the textarea tags -> print("Namaste World"),

The second code block was added manually and it does have data but it's not rendered in the html -> we are passing it in the value attribute of textarea.

I suspect this might be the reason why are are not able to navigate with cursor and also can't delete the data. If we manually remove that from html (dev tools), it works, which proves we are suspecting the right reason.