feat: add app-aware router creation

[armanmikoyan]

Description

Closes #2564.

Adds app.createRouter(options) to create an app-aware router that inherits the application's applicable routing settings:

• case sensitive routing
• strict routing

Routers created with express.Router() do not inherit these settings:

const app = express()

app.enable('case sensitive routing')
app.enable('strict routing')

const router = express.Router()
// Does not inherit the app settings
// caseSensitive: undefined
// strict: undefined

The new method allows these settings to be inherited:

const router = app.createRouter()
// caseSensitive: true
// strict: true

Explicit options override inherited settings, while other Router options are passed through:

const router = app.createRouter({
  strict: false,
  mergeParams: true
})

Tests

• Added tests for inherited routing settings
• Added tests for explicit overrides
• Added coverage for additional Router options
• Router-related test suite passes