Problem:
tostring(vim.version()) returns "table: 0x…".

Solution:
Modify vim.version() to return a string prerelease instead of a boolean.

Fix #23863

vim.version.range() couldn't parse them correctly.
For example, vim.version.range('<0.9.0'):has('0.9.0') returned `true`.

fix: range:has() accepts vim.version()
So that it's possible to compare a range with:

    vim.version.range(spec):has(vim.version())

Problem:
semver specifies that digit sequences in a prerelease string should be
compared as numbers, not lexically: https://semver.org/#spec-item-11
> Precedence for two pre-release versions with the same major, minor,
> and patch version MUST be determined by comparing each dot separated
> identifier from left to right until a difference is found as follows:
> 1. Identifiers consisting of only digits are compared numerically.
> 2. Identifiers with letters or hyphens are compared lexically in ASCII sort order.
> 3. Numeric identifiers always have lower precedence than non-numeric identifiers.
> 4. A larger set of pre-release fields has a higher precedence than a smaller set, if all of the preceding identifiers are equal.
Example:
1.0.0-alpha < 1.0.0-alpha.1 < 1.0.0-alpha.beta < 1.0.0-beta < 1.0.0-beta.2 < 1.0.0-beta.11 < 1.0.0-rc.1 < 1.0.0.

Solution:
cmp_prerel() treats all digit sequences in a prerelease string as
numbers. This doesn't _exactly_ match the spec, which specifies that
only dot-delimited digit sequences should be treated as numbers...

Problem:
"tmux 3.2a" (output from "tmux -V") is not parsed easily.

Solution:
With `strict=false`, discard everything before the first digit.

- rename Semver => Version
- rename vim.version.version() => vim.version._version()
- rename matches() => has()
- remove `opts` from cmp()

Now the Nvim version string "v0.9.0-dev-1233+g210120dde81e" parses
correctly.

Use semver code from https://github.com/folke/lazy.nvim
License: Apache License 2.0

Co-authored-by: Folke Lemaitre <folke.lemaitre@gmail.com>

Problem:
If major<major but minor>minor, cmp_version_core returns 1

Solution:
- Fix logic in cmp_version_core
- Delete most eq()/gt()/lt() tests, they are redundant.

- version.cmp(): assert valid version
- add test for loading vim.version (the other tests use shared.lua in
  the test runner)
- reduce test scopes, reword test descriptions