Fix nested path issue in Swagger and Scalar config

[chrisllontop]

This PR fixes the issue reported on #135

[LhonRafaat]

When is this gonna release?

[TheGU]

When we set the prefix for Elysia, this change breaks the swagger user interface.

Minimal code to test that was functional before to this update and will malfunction once it is implemented.

import { Elysia } from 'elysia'
import { swagger } from '../src/index'

const app = new Elysia({ prefix: '/api' })
    .use(swagger())
    .get('/id/:id?', 'a')
    .listen(3000)

The issue is that it appends "/" to the beginning of the route, making it begin at the root rather than a relative. Therefore, when we open the Swingger UI, it attempts to retrieve the openapi json from
http://localhost:3000/swagger/json
, but it cannot find it. It should be
http://localhost:3000/api/swagger/json .
This is because it removes the relative path and starts with from root with url start with "/"

This might be relate to #183 , #162 , #151
It should also solve a problem for #154