vrepl: fix os.input() (fix #5678)

[yuyi98]

This PR fix os.input() in vrepl tools (fix #5678).

• Fix os.input() in vrepl tool.
• Add test.

PS D:\Vlang\v> v
 ____    ____
 \   \  /   /  |  Welcome to the V REPL (for help with V itself, type  exit , then run  v help ).
  \   \/   /   |  Note: the REPL is highly experimental. For best V experience, use a text editor,
   \      /    |  save your code in a  main.v  file and execute:  v run main.v
    \    /     |  V 0.4.6 4475759 . Use  list  to see the accumulated program so far.
     \__/      |  Use Ctrl-C or  exit  to exit, or  help  to see other available commands.

>>> mut s := os.input('aaa> ')
aaa> hello
>>> s
hello
>>> s = os.input('bbb> ')
bbb> world
>>> s
world
>>>

[spytheman]

Excellent work.