feat: add type alias support

[klaidliadon]

Adds type alias rendering as JS comments. Since JS has no type system, aliases are emitted as `// type Name = BaseType` comments in the generated output.

Changes

• `types.go.tmpl`: Added `isAliasType` check block between enum and struct sections
• `_examples/hello-webrpc/hello-api.ridl`: Added `Username` and `Age` alias examples
• `_examples/hello-webrpc/webapp/client.gen.js`: Regenerated with alias comments

Generated output example

```js
// type Username = string
// type Age = uint32
```