Problem: Repeated default fields and field-by-field assertions obscure
the values that each functional test is meant to check.

Solution: Add eq_partial() to check selected record fields while
preserving list length and order. Use local helpers to fill fixed
expected defaults where tests need full equality. Keep explicit absence
checks and test-specific values at the call sites.

AI-assisted

Problem:
An inherited Git index can leave a plugin checkout empty or prevent
updates.

Solution:
Clear GIT_INDEX_FILE for plugin Git commands. Verify installs and
updates preserve the caller environment and index.

AI-assisted
Signed-off-by: Jake Wang <jakezwang@users.noreply.github.com>

Problem: No way for plugins to define hooks that would be executed
  during plugin lifecycle. Like after install/update or before
  update/delete.

Solution: Automatically source scripts defined in plugin manifest file
  (if any) after triggering corresponding `PackChanged{,Pre}` events.
  During sourcing make some termporary adjustments:
  - Current directory is set to plugin's root to simplify execution of
    CLI calls like `make build`.
  - Plugin's path is ensured to be inside 'runtimepath' to allow using
    `require('plugin-module')` inside manifest scripts.

  The reason to execute after triggering event is so that there is a
  possibility for users to execute code both before the script (exactly
  on event) and after the script (more-or-less via `vim.schedule` called
  on the event).

Problem: There is no agreed way for plugins to provide extra information
  about themselves. Like required Neovim version, install/update/delete
  hooks, and dependencies.

Solution: Introduce basic concept of plugin manifest file `pkg.json`
  based on https://packspec.org/.

Problem: Installing plugins during lockfile synchronization always
  writes the lockfile, even though its content is used during install.
  This might be a problem if the lockfile (itself or its parent
  directory) is not writeable (can only be read).

Solution: Do not write the lockfile when installing directly from it.
  This is okay since the `src` and `rev` are used directly from the
  lockfile and don't change at this step. While potential change in
  `version` (that must be written to the lockfile) is handled in other
  code path.

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`.

    FAILED   …/plugin/pack_spec.lua:92: …/plugin/pack_spec.lua @ 70: vim.pack update() can use lockfile revision as a target
    …/plugin/pack_spec.lua:92: …/plugin/pack_spec.lua:70: process still exists
    stack traceback:
    D:/a/neovim/neovim/test/functional/testnvim/exec_lua.lua:124: in function 'system_sync'
    …/plugin/pack_spec.lua:92: in function 'git_cmd'
    …/plugin/pack_spec.lua:113: in function 'git_add_commit'
    …/plugin/pack_spec.lua:1209: in function <…/plugin/pack_spec.lua:1173>

fix #40167

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>