Implement JsLifetime for ()

[Sytten]

We need that for https://github.com/rquickjs/rquickjs-moduledef-ext to allow users to not have options

use rquickjs_moduledef_ext::{ModuleLoader, ModuleDefExt, ModuleImpl};

struct MyModule;

impl ModuleDefExt for MyModule {
     type Implementation = ModuleImpl<()>;

     fn implementation() -> &'static Self::Implementation {
         &ModuleImpl {
             declare: |decl| {
                 decl.declare("hello")?;
                 Ok(())
             },
             evaluate: |ctx, exports, options| {
                 exports.export("hello", "world".to_string())?;
                 Ok(())
             },
             name: "my-module",
         }
     }

     fn options(self) -> () {}
 }

 let (loader, resolver, initializer) = ModuleLoader::builder()
     .with_module(MyModule)
     .build();

[DelSkayn]

Looks good!