Operator to apply string interpolation on value similar to envsubst

[jedvardsson]

I suggest to add an operator similar to envsubst to explicitly invoke the new string interpolation feature on a value.

For example,

color: green
fruit: '\(color) apple'

yq '.fruit |= interpolate' sample.yaml

Could output

color: green
fruit: 'green apple'

Note that the uninterpolated cannot use double quotes since \( is not a valid escape sequence.

Here is an example using envsubst:

fruit: '${color} apple'

color=green yq '.fruit |= envsubst' sample.yaml

outputs

fruit: 'green apple'