Is it possible to define a native function with a default argument?

Jsonnet's function supports default argument.

https://jsonnet.org/learning/tutorial.html#functions

e.g.

```jsonnet
// Default arguments are like Python:
local my_function(x, y=10) = x + y;
```

https://pkg.go.dev/github.com/google/go-jsonnet@v0.20.0#NativeFunction

NativeFunction has a field `Params`, which a list of Identifier.
Identifier represents a variable / parameter / field name. +gen set.

https://pkg.go.dev/github.com/google/go-jsonnet@v0.20.0/ast#Identifier

I want to define a default argument, but is it possible?

I tried to add a default argument named `opts`, but it didn't work.

```go
Params: ast.Identifiers{"pattern", "s", "opts"},
```

If I omit the argument, the error `RUNTIME ERROR: Missing argument: opts` occurred.

I tried the following code, but it also didn't work. `RUNTIME ERROR: Missing argument: opts={}`

```go
Params: ast.Identifiers{"pattern", "s", "opts={}"},
```