feat: support a function as the error message

[gmaclennan]

Add support for passing a function as the message of an error definition. The function receives the (typed) params object and returns the message string. The params type for the constructor is inferred from the function's parameter, so you get full type-checking without a {param} template.

const TooMany = createErrorClass({
  code: "TOO_MANY_ITEMS",
  message: (params: { items: number[] }) =>
    `Found ${params.items.length} items, expected fewer`,
  status: 400,
});

new TooMany({ items: [1, 2, 3] });                  // ✅ params typed as { items: number[] }
new TooMany({ items: [1, 2, 3] }, { cause: err });  // ✅
new TooMany({ items: "nope" });                     // ❌ caught at compile time

A zero-argument function behaves like an error without template parameters:

const Unauthorized = createErrorClass({
  code: "UNAUTHORIZED",
  message: () => "Access denied",
  status: 401,
});

new Unauthorized();

Params type is inferred from the function's first parameter; whether params are required is driven by the function's
arity (number of arguments it expects - () => and (params = {}) => are no-param, (params) => requires params), keeping the runtime and type levels consistent.

Note: ErrorDefinition's default type parameter widens from string to string | MessageFn in this release. This is a public type change but is treated as non-breaking — ErrorDefinition is intended for internal use by this library, and external consumers reading def.message as a string would be unusual. Flagging in case anyone depends on it.