Fixes #3362.

* Add new tarantool console directive '\':

  \set language <language> for setting language
  \set delimiter <delimiter> for setting delimiter
  \help show this screen
  \quit quit interactive console

* Add support for SQL language

See #2117

Revised the entire tutorial (file help_en_US.lua):
 * Tested and fixed all sample code snippets.
 * Corrected typos and inconsistent names in screen headers.
 * Updated info about custom delimiters (screen #6).
 * Replaced box.session with fiber (example on screen #10).
 * Swapped screens #11/#12 (fibers/sockets) for smoother story flow.

Removed help-related stub logic (file help.lua).

Updated auto-tests to expect a new output result on help call.