[bug] option function type definition is different from yargs

[gwsbhqt]

https://github.com/microsoft/just/blob/dc1c033ee1a49c1ee85a9c5897ae337dc1d9b619/packages/just-task/src/option.ts#L24

Just 's option function define the "default" member as object type, but at yargs example code, the "default" member is primitive type.

https://github.com/yargs/yargs/blob/59a86fb83cfeb8533c6dd446c73cf4166cc455f2/example/complex.js#L9

In practice, I did encounter type problems.

I would suggest changing the type to any or unknown, thanks.