Add tty_inkey(),tty_simple() and tty_subcmd_readln()

Add following functions to handle input from a thread that uses standard
input.

tty_inkey() waits for a key to be pressed for a specified amount of
time, and then returns the pressed string.

tty_simple_readln() reads user input until Enter is pressed.
It supports Backspace key.

tty_subcmd_readln() reads user input until Enter is pressed.
It supports line editing (cursor keys, Backspace) and command history.

The script REPL history buffer is now a dedicated history buffer instead
of the subcommand history buffer. REPL prompt changes from ">>" to "->".

Reduced the history buffer size from 1000 lines to 500 lines.

docs/interactive-lua-scripting.md

@@ -191,13 +191,13 @@ Example:
 [10:17:30.215] Enter file name or "!" Lua commands or "@" interpreter directive:
 >> @repl
 [10:17:31.956] Enter Lua REPL mode (@exit to exit)
->> p=1
->> for i=1,10 do\
->>     p = p * i\
->> end
->> print(p, "\r")
+-> p=1
+-> for i=1,10 do\
+->     p = p * i\
+-> end
+-> print(p, "\r")
 3628800
->> @exit
+-> @exit
 ```
 
 ---