Don't run empty commands (including comments)

[ageron]

This PR makes sure the REPL ignores empty commands, including comment-only commands.
This is useful if you want to give yourself some visual space by typing Enter several times.
It's also useful if you want to write some comments for educational purposes.

Before

»                [I just typed Enter here]

Enter an expression to evaluate, or a definition (like x = 1) to use later.

  - ctrl-v + ctrl-j makes a newline
  - :q quits
  - :help shows this text again

» # You can do plenty of things in the Roc REPL,
… # including arithmetic:

Enter an expression to evaluate, or a definition (like x = 1) to use later.

  - ctrl-v + ctrl-j makes a newline
  - :q quits
  - :help shows this text again

» 2 * 3

6 : Num *
» 

After

»

» # You can do plenty of things in the Roc REPL,
… # including arithmetic:

» 2 * 3

6 : Num *

I also replaced line.trim() with line.trim_end(), because the spaces don't hurt, and sometimes you want to keep those spaces for visual purposes, for example:

»         ############
… 2 * 3   # AMAZING! #
…         ############

6 : Num *

[ageron]

I'm not sure why this PR is failing, it looks like the failure is due to some other change, unrelated to this PR. Wdyt?

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