Problem:
A trailing separator in 'shell' leaves the parsed end before the path
tail. The resulting length underflows and crashes option initialization.

Solution:
Reset the end pointer when advancing past a separator, preserving quoted
path parsing. Cover a trailing separator at startup.

AI-assisted

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:
* 'shellcmdflag' states that its default value is set according to the
  value of 'shell', but this behavior is not yet implemented on Windows.
  The same applies to 'shellpipe', 'shellredir', and 'shellxquote'.
* On Windows, Git is often installed in paths containing spaces, and we
  still do not correctly resolve the sh executable name as described in
  'shell'.
* On Windows, the default value of 'shellslash' is always `false`,
  which causes Unix-like shells to interpret `\` in paths returned by
  some functions as escape charaters.

Solution:
Use a simple rule table to detect common shells (e.g. `cmd`,
`powershell`, shells whose names contain `csh` or `sh`) and apply
best-effort defaults, while leaving more complex scenarios to user
configuration.