Without this the vscode extension cannot find the example executable, and the example fails.

Also use $USERPROFILE instead of  $HOME for finding the `.vscode` user directory for installing the extension.

Bug: #12

Empty structs were being serialized as `null`, when they should have been serialized as `{}`.

This was due to the type inference on the serializer - where no calls to `field()` would result in the default `null` type.

To solve this, the `serialize(const void* object, const std::initializer_list<Field>&)` inline helper has been promoted to a virtual function (and renamed to `fields()`).
The JSON serializer implementation of this now first sets the object type to `object`, even if there are no fields to serialize.

Added test for this.

Fixes: #10

Compiler warning was signed / unsigned compare.

The `body` field of the Response is optional. Do not error if it is missing.

If the typeinfo of the response. This was incorrectly using the Request type.

Authored by kuafuwang, squashed by ben-clayton.

The onError parameter of Server::start was default initialized with
OnError(), which is an 'invalid target', not a no-op implementation.

Replace with a true no-op implementation.

`nlohmann/json` raises a `size_t` to `int` MSVC warning that will fail the build if `CPPDAP_WARNINGS_AS_ERRORS` is enabled.
Given that this is not our code, the best we can do is to make an exception for this warning.

Only set the third_party directory variables if they're not already set
(by the dependee project).