In an update to Scratch 3.0 coming soon to a scratch.mit.edu near you, the enter key can be discussed by the key-pressed block! It doesn't show up in the menu, but by using the "join" block (or a variable reporter), we can insert that name into the input and it will work! No /guarantees/ this is added since it's coming in a future update and the ST may find that it's not quite ready to be released after they do testing, buuut... if it gets released, you can find out with this project! :) Only enter is new - still no support for shift, control, etc. Those are special accessibility-related keys, so it's not very likely that Scratch will ever see them added, but who knows. :P Have a look at the discussion of this new feature here: https://github.com/LLK/scratch-vm/pull/2247 (If you look closely, you might be able to find out how to test out this out right away, without waiting for the update... >:D)
