[js/webnn] Enable user-supplied MLContext (#20600)
### Description
This PR enables the API added in #20816 as well as moving context
creation to JS.
### Motivation and Context
In order to enable I/O Binding with the upcoming
[MLBuffer](https://github.com/webmachinelearning/webnn/issues/542) API
in the WebNN specification, we need to share the same `MLContext` across
multiple sessions. This is because `MLBuffer`s are restricted to the
`MLContext` where they were created. This PR enables developers to use
the same `MLContext` across multiple sessions.
