Allow user to add own snippets

[ElSargo]

So often in shaders, we use very short variable names to speed up typing. This works but the short names make it hard to understand the code. I would love to use longer, more descriptive names but this would make typing it out many times without making typos a pain.

Clearly, the best solution would be to analyze the code in realtime and to add the names of variables and functions to the snippets, but this would be a massive challenge to implement. An alternative could be to use a hotkey to add the selected text to the snippets for auto-complete.