bug: Support inheritance

It looks like `theia-vscodecov` doesn't properly support inheritance.
Can be reproduced with `gitlab-vscode-extension-main`:

* `theia-vscodecov` reports `Selection.start` as missing even though it is inherited from `Range.start` which is supported
* Gitlab extends `vscode.Comment` in its code base and `theia-vscodecov` reports all members it adds as missing