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:
Continue separators normalization, and try to keep it at the
nvim <-> external boundary, e.g., `fn.xxx`, `api.xxx`, `:xxx`

Solution:
some key changes
- normalize `$HOME-windows`
- normalize entry points of changing directory
  - `nvim_set_current_dir()`, `chdir()`
- normalize the named pipe
  - `--listen` arg, `--server` arg, `:restart`
  - `serverstart()`, `sockconnect()`, `serverstop()`
- make `expand()` respect 'shellslash' again
- clean up `did_set_shellslash`
- replace `forward_slash` with `TO_SLASH`
- remove obsolete `TMP_PATHSEPSTR`
- remove `slash_adjust` in `do_autocmd_dirchanged`?
- make `fnamemodify()` always return `/` (except when substituting
  separators via `:s`, `:gs` ?)

Note:
- these funcs still apply `slash_adjust` on return, as before:
  `:pwd`, `chdir()`, `exepath()`, `getcwd()`
- these funcs alwarys return `/`, unlike before the normalization PRs
  - `getcompletion()`, `finddir()`, `findfile()`

Also
- clean up code, comments, formatting and tests
- move `TO_SLASH` from `f_chdir` to `changedir_func`
- move `TO_SLASH` from `f_bufadd` to `buflist_new`

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

**Problem:**
The brackets in the RFC2732 regular expression are currently unescaped,
causing them to be misinterpreted as special characters denoting
character groups rather than as literal characters.

**Solution:**
Escape the brackets.
Fix #31270

- `alter_slashes` belongs in `testutil.lua`, not `testnvim.lua`.
- `alter_slashes` is an unusual name. Rename it to `fix_slashes`.
- invert its behavior, to emphasize that `/` slashes are the preferred,
  pervasive convention, not `\` slashes.

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.

Work on https://github.com/neovim/neovim/issues/27004.

Problem: Some LSP servers return `textDocument/documentLink` responses
         containing file URIs with line/column numbers in the fragment.
         `vim.uri_to_fname` returns invalid file names for these URIs.

Solution: Remove the URI fragment from file URIs.