Improve ReSharper extension build and upload

Currently, the build process for the ReSharper extension happens as part of the (internal) TeamCity CI build. It will create artifacts for both the Rider (bundled) plugin and the `.nupkg` file to upload for ReSharper. The process can be improved:

* The type of build should be reflected in the `.nupkg` name and version. E.g. if it's an EAP build, we should append `-eap`, so we can use the EAP channel in the ReSharper plugins gallery.
* Investigate automatically publishing the EAP ReSharper builds. There is no need to publish more frequently.
* Investigate automatically publishing RTM builds.
* Investigate if we can also have nightly builds. Due to the way the gallery works, it doesn't support prerelease in the same way that NuGet does. The trailing data in the version (e.g. the `-eap` in `2020.3.0.2-eap.nupkg`) is the release channel, not a preview version. For EAP builds, this needs to be just `-eap` and not a versioned `-eap` such as `-eap7`. Versioning is solely handled by the version number (and reflects 4 digits, while NuGet/semver only really expect 3). Should normal non-EAP builds be suffixed with `-nightly`? How does this affect ReSharper's Extension Manager? Will it see `-eap` and `-nightly` as the same, i.e. "pre-release", but with different preview versions?