Should we allow struct expressions?

[JLHwung]

In #38 we removed the struct expressions for grammar conflict case, however, I do believe the non-shared StructExpression would be useful:

For example it can serve as a parameter initializer:

function parse(
  input,
  options = struct DefaultOption {
    static scriptType = guessScriptType(input);
    static version = 'es2025';
  }
) { }

Note that because the DefaultOption struct depends on the input, it can't be hoisted to the top level. A workaround is to move it into the function body, which basically desugars the parameter initializer, plus we can't use ternary operator because struct expression is currently not a thing:

function parse(
  input,
  options
) {
  if (options === undefined) {
    struct DefaultOption {
      static scriptType = guessScriptType(input);
      static version = 'es2025';
    }
    options = DefaultOption;
  }
}

In practise, the options could be accessed frequently, so a fixed layout will definitely benefit the performance, and avoid performance regression from polymorphism.