Enable `declaration-property-value-no-unknown`

[romaricpascal]

Stylelint 15 adds an optional new rule declaration-property-value-no-unknown to check for invalid property/value combinations such as:

.govuk-link--example {
  top: red;
}

The new rule looks very useful, but may require careful configuration to ensure it works across the different projects using stylelint-config-gds and accepts things like functions for projects using Sass.

For example, turning it on by default will throw errors for unknown values govuk-colour("green")

// Disallow unknown property and value pairs
// https://stylelint.io/user-guide/rules/declaration-property-value-no-unknown/
'declaration-property-value-no-unknown': true

For which we might ignore all govuk-* values:

'declaration-property-value-no-unknown': [
  true, {
    ignoreProperties: {
      '/.+/': '/govuk\\-/'
    }
  }
]

Or alternatively investigate the newer stylelint-scss rules instead