[RFC] version refactor

ref #1135 #1266

- Changed cosmetic version strings (e.g., `VIM_VERSION_LONG` => `NVIM_VERSION_LONG`) to use nvim versions and verbiage. These are user-facing only, and don't affect plugins.
- Functional version identifiers (`VIM_VERSION_MAJOR`/`MINOR`/`100`) will continue to track upstream Vim for plugin compatibility.
- Introduce `NVIM_VERSION_PRERELEASE` and `NVIM_VERSION_BUILD` as optional strings controlled by cmake or `CFLAGS`.
- Rather than add another vimscript mechanism such as `v:nversion` or something, I propose we enable users to check the nvim version via `has("nvim-1.2.3")`. (I stubbed this, but didn't write the logic yet.)

Future considerations:

- does API version need to be separate from Nvim version? Maybe not. Because any breaking API changes will require incrementing the major-version, anyways.
- `NVIM_VERSION_BUILD` should not be set at all for release build, but we would need to establish some sort of flag that indicates this, perhaps in #1071. Otherwise we are stuck checking for various build types (MinSizeRel, RelWithDebInfo, Release, ...).
Until we reach a release, this will always report `0.0.0-alpha+<commit sha>`.