orDefault function

[TomWright]

Add a new function orDefault.

It takes 2 arguments:

• A selector string
• A selector string that will be used as a fallback

The first selector is expected to return a single value.
The second selector is used if the first causes a NotFound error and is expected to always return a single value.

Example usage:

$ echo '{
  "-LCr5pXw_fN32IqNDr4E": {
    "bookCategory": "poetry",
    "locale": "en-us",
    "mediaType": "book",
    "publisher": "Pomelo Books",
    "title": "Sound Waves",
    "type": "poetry"
  },
  "-LDDHjkdY0306fZdvhEQ": {
    "ISBN13": "978-1534402966",
    "bookCategory": "fiction",
    "title": "What Can You Do with a Toolbox?",
    "type": "picturebook"
  }
}' | dasel -r json 'all().mapOf(projectId,key(),title,title,bookCategory,bookCategory,type,type,locale,orDefault(locale,string(unknown)))'
{
  "bookCategory": "poetry",
  "locale": "en-us",
  "projectId": "-LCr5pXw_fN32IqNDr4E",
  "title": "Sound Waves",
  "type": "poetry"
}
{
  "bookCategory": "fiction",
  "locale": "unknown",
  "projectId": "-LDDHjkdY0306fZdvhEQ",
  "title": "What Can You Do with a Toolbox?",
  "type": "picturebook"
}