Create 2.0.0 Pre-release

[AldaronLau]

Eventually, there will need to be some breaking changes, including bumping the MSRV to 1.65 / upgrade to the 2021 edition. My current plan is to have several pre-releases of 2.0.0 before stabilization, for some pre-testing (since it's planned as a LTS release, with support until at least the release of the 2027 edition). The 2.0 pre-release track will likely be it's own non-default branch before being merged (for 2.0.0 release), after which possibly 1.0 track will become it's own branch and receive any critical patches.