Prepare pre-release 2.1.0

[fendor]

Additionally, add a release checklist for future contributors.

Most notably, this version is changed to 2.1.0 to follow the version recommendation at https://code.visualstudio.com/api/working-with-extensions/publishing-extension#prerelease-extensions

... we recommend that extensions use major.EVEN_NUMBER.patch for release versions and major.ODD_NUMBER.patch for pre-release versions. For example: 0.2.* for release and 0.3.* for pre-release.

with the reasoning:

VS Code will auto update extensions to the highest version available, so even if a user opted into a pre-release version and there is an extension release with a higher version, that user will be updated to the released version.

So, the next proper release is going to be 2.0.1, and the next pre-release is 2.1.0. Unless there is a strong incentive to do so, we won't yank the pre-release 2.0.0, mainly because it feels like unnecessary work.