Hide API implementation proxies, add tests to check for "proxy leakage"

[jrieken]

Our vscode API is mostly implemented through RPC'ing to the renderer. That's done through a protocol implementation and objects that represent a slice of the API, aka proxies, like MainThreadTextEditorsShape. These proxies usually mirror the capabilities of the API but are often more powerful.

This PR adds marker properties to proxies and the rpc-objects. It also adds a test-util that tries to reach these proxies using the API instance as entry point. In the name of being very strict, it's a test failure when any rpc object can be reached.

Note that the test can only checks on existing objects, e.g if vscode.window.activeTextEditor is undefined it cannot be traversed. That's why a check is performed during teardown of a single test. That increases the coverage but doesn't guarantee that everything is checked.

The PR also changes how some APIs are implemented (so that proxies aren't accessible anymore). There is two strategies

1. Use a true private property, e.g #proxy, so that reflection doesn't find them. However, note that the emitted polyfill is significantly slower
2. Use an object that only implements the API shape and has access to "internals" via scope variables. I have done that in most cases via the a value property, e.g ExtHostTerminal#value. There is another benefit to this approach which is TS checking that the literal only implements what is defined in the API, so accidental API leakage is checked by the compiler

• fyi @Tyriar for changes in terminal land (see 3d2ca29)
• fyi @alexr00 for changes in tasks land (see 82c629e)
• fyi @weinand for changes in debug land (see 3114b1c)

[Tyriar]

Nice work, terminal looks good 👍

[alexr00]

Tasks looks good, thank you!