console: configure continuation

Continuation marker can be set up with `\set continuation` command.
Works on both server and client side in any language.

Closes #4317
Requires #7357

@TarantoolBot document
Title: introduce line carrying slash

Now we can use multiline commands with lines ending by configuring
continuation symbol. Works only when there is no set delimiter.
Consider the example where the marker is set, used and removed:
```
tarantool> \set continuation on
---
- true
...

tarantool> a = 10\
         > + 12
---
...

tarantool> \set continuation off
---
- true
...

tarantool> a = 10\
---
- error: '[string "a = 10\"]:1: unexpected symbol near ''\'''
...

tarantool>

```