Resource Alias Support in ServletContextHandler

**Jetty version(s)**
12.1

**Enhancement Description**

In an embedded Jetty 12.1 setup, a `ServletContextHandler` is being used to configure multiple servlets, but it does not provide the option to define any alias. But a `WebAppContext` provides the method `setResourceAlias()` to register resource aliases, which would address this need. Adding support for resource aliasing in the `ServletContextHandler` would enable more flexible resource mapping in embedded Jetty configurations without requiring the use of WebAppContext.

see discussion https://github.com/jetty/jetty.project/discussions/13527

Start at ServletContextHandler and compare its resource handling with WebAppContext.setResourceAlias(). Read the linked discussion for the expected alias behavior, then identify the relevant ServletContextHandler tests or test entry points. Done means embedded Jetty configurations can define resource aliases without switching to WebAppContext, with behavior covered by tests.

Written by the indexing model from the issue text.