Quote all string values in EXPLAIN (FORMAT YAML) output.

While my previous attempt seems to always produce valid YAML, it
doesn't always produce YAML that means what it appears to mean,
because of tokens like "0xa" and "true", which without quotes will
be interpreted as integer or Boolean literals.  So, instead, just
quote everything that's not known to be a number, as we do for
JSON.

Dean Rasheed, with some changes to the comments by me.