Problem:
`vim.version.intersect()` returns tables with `VersionRange` (method
table) as their metatable. Although the calculated bounds are correct,
the results do not expose `VersionRange` methods.

Solution:
Construct intersection tables with `range_mt`, matching
`vim.version.range()`. Add assertions covering method access and
repeated intersection.

Problem:
The magic globals `it`, `describe`, etc., are more trouble than they are
worth.

- Hooking into `after_each` requires `getfenv()` hacks.
- They confuse luals/emmylua, because the top-level `.luarc.json` isn't
  merged with `test/.luarc.json` (apparently a luals limitation?)
- They totally defeat discoverability because the user just has to
  "know" about the various magic symbols.

So they harm DX, which means they serve no purpose at all.

Solution:
- Expose the test API from `testutil`, so tests can call `t.it()`,
  `t.describe()`, etc., in the conventional way.
- Drop `getfenv()` hacks.
- Drop the `setfenv()` injection in `load_chunk`.
- Drop `test/_meta.lua`.

Problem: vim.VersionRange had no __eq metamethod, so comparing 2 distinct
but same value instances always returned false. In vim.pack.add this caused
redundant lockfile rewrites, even when the resulting lockfile content was
unchanged.

Solution: Add __eq metamethod on vim.VersionRange

Problem: No way to compute intersection of two version ranges, which is
useful when computing version range that fits inside several reference
ranges.

Solution: Add `vim.version.intersect()`.

Problem: `vim.version.range('<=a.b.c')` is not precise when it comes to
its right hand side. This is due to version ranges using exclusive right
hand side. While `vim.version.range('>a.b.c')` is not precise when it
comes to its left hand side because left hand sides are inclusive.

Solution: For '>=a.b.c' increase `to` from 'a.b.c' to the smallest
reasonable version that is bigger than 'a.b.c'. For '<a.b.c' do the same
for `from`.
More proper solution is an explicit control over inclusivity of version
range sides, but it has more side effects and requires design decisions.

Problem: `tostring()` applied to version range doesn't return
human-readable text with information about the range.

Solution: Add `__tostring()` method.

Problem:
`vim.version.range('>=0.10'):has('0.12.0-dev')` returns false, which is
wrong per semver.

Solution:
`vim.VersionRange:has()` shouldn't have special handling for prereleases
(Why would we need it when `__eq`, `__lt`, `__le` already handle
 prereleases?).

Closes #33316

Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.

Closes https://github.com/neovim/neovim/issues/27004.