document \set output lua

[TarantoolBot]

Historically we use YAML format to print results of operation to
a console. Moreover our test engine is aiming YAML as a primary format
to compare results of test runs. Still we need an ability to print
results in a different fasion, in particular one may need to use
the console in a REPL way so that the results would be copied and
pased back to further processing.

For this sake we introduce that named "output" command which allows
to specify which exactly output format to use. Currently only yaml
and lua formats are supported.

To specify lua output format type

| tarantool> \set output lua

in the console. lua mode supports line oriented output (default) or
block mode.

For example

| tarantool> a={1,2,3}

tarantool> a
- - 1
- 2
- 3
...
tarantool> \set output lua
true
tarantool> a
{1, 2, 3}
tarantool> \set output lua,block
true
tarantool> a
{
1,
2,
3
}

By default YAML output format is kept for now, simply to not
break the test engine. The output is bound to a session, thus every
new session should setup own conversion if needed.

Since serializing lua data is not a trivial task we use "serpent"
third party module to convert data.

Part-of #3834
Requested by @cyrillos in tarantool/tarantool@4272550.

[pgulutzan]

I added a section "Interactive console" describing the \set instructions including this one.