[wasm][debugger] Show actual data for boxed values (#41562)

[wasm][debugger] Show actual data for boxed values (#41562)

* [wasm][debugger] Add support for Nullable<T>

Return the value, or null.

Fixes https://github.com/mono/mono/issues/20310

* Address review feedback - merge functions

* [wasm][debugger] run dotnet-format on the debugger test app

* [wasm][debugger] simplify function sig, based on usage

- addresses review feedback from @lewing

* [wasm][debugger] Simplify the function further, based on @lewing's

.. excellent suggestion!

* [wasm][debugger] Show actual data for boxed values

Eg. `object o = "foobar"`

This will show the string `"foobar"`, instead of an object, in the
debugger.