Double quote YAML strings

[daniel-shuy]

config-merge uses YAML.stringify() to format the merged output as YAML.

While YAML.stringify() outputs YAML version 1.2 by default (https://eemeli.org/yaml/#document-options), the merged output may be consumed/parsed by other applications that expect YAML version 1.1. This is normally not an issue, except when it comes to the yes/no strings. By default, YAML.stringify() formats strings unquoted. YAML version 1.2 parses unquoted yes/no strings as strings, while YAML version 1.1 parses unquoted yes/no strings as true/false booleans (https://eemeli.org/yaml/#version-differences).

An easy solution is to configure YAML.stringify() to double-quote all strings by default, by setting defaultStringType to QUOTE_DOUBLE (https://eemeli.org/yaml/#tostring-options).

[daniel-shuy]

Hi @matthewdevenny, could you help to take a look at this PR? Thanks!