Parser functions do not support named parameters the way templates and tag extensions do, but it is occasionally useful to fake it.
For uniformity of template syntax parsing, and to prevent continued slightly-incompatible wheel reinvention, we should fix this. When a parser function is registered with SFH_NAMED_ARGS as a flag to Parser::setFunctionHook standardized parsing of named arguments should be done by core. Users should be encouraged to migrate to this.
We should think carefully through the desired behavior for whitespace-stripping of argument values, and ensure it makes sense with T114432: [RFC] Heredoc arguments for templates (aka "hygienic" or "long" arguments).