Feature: #103671 - Provide null coalescing operator for TypoScript constants

See forge#103671


TypoScript constants expressions have been extended to support a null coalescing operator (??) as a way for providing a migration path from a legacy constant name to a newer name, while providing full backwards compatibility for the legacy constant name, if still defined.

Example that evaluates to $config.oldThing if set, otherwise the newer setting $myext.thing would be used:

plugin.tx_myext.settings.example = {$config.oldThing ?? $myext.thing}


Since Feature: #103439 - TypoScript provider for sites and sets it is suggested to define site settings via settings.definitions.yaml in site sets instead of TypoScript constants. Migration of TYPO3 Core extensions revealed that such migration is a good time to revisit constant names and the null coalescing operator helps to switch to a new setting identifier without breaking backwards-compatibility with previous constant names.