Add missing variablesReference field to DAP evaluate request

[rsubtil]

Fixes #89052

As @dekaravanhoc noted, the DAP spec for the evaluate request requires a variablesReference field to be set.

Since Godot fully parses the given expression, this field is hardcoded to 0 according to the spec behavior:

/**
* If variablesReference is > 0, the evaluate result is structured and its
* children can be retrieved by passing variablesReference to the
* variables request as long as execution remains suspended. See 'Lifetime
* of Object References' in the Overview section for details.
*/

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.2.2.