Use quotes in prettify output

[hagen1778]

With #40 metricsql package now supports quoted metric names {"metric_name"}. It also allows using arbitrary UTF8 chars within the quotes: {"3foo3"}. However, the prettify functionality would transform {"3foo3"} in \3foo\3.
It would be great, for user's convenience, to keep quotes instead of escaping. The expected test cases for prettyfying are the following:

• {"metric"} => metric
• {"3metric"} => {"3metric"}
• {"met\"ric"} => {"met\"ric"}

[possibull]

Also the case of using any metric name in prettify will fail by stripping the quotes ie {"foo"} will become {foo} which is invalid.