Problem:
There is no unified notion of a "user action".

Vim processes input by one-char-at-a-time, and mostly throws away any
hints it might gather about the user's action, with one exception: it
stores the last _edit_ action (the "redo buffer", encoded as
unstructured `["x][v][count]body` bytes).

Plugins can only observe individual keys (vim.on_key) and high-level
effects (TextChanged, CursorMoved).

Solution:
- Users can subscribe to `CmdAtom` events to handle any user action.
  - Event is deferred; handlers cannot cancel or interfere with user
    actions.
- Capture `CmdSpec` from the normal/insert/visual subsystems.
  - typeahead/readahead stay unstructured (`buffheader_T`): they are key
    streams, not commands.
  - the redo/record buffers become `StringBuilder`: fewer
    allocations/copies.
- Repurpose the input/redo engine to accept `CmdSpec` objects.

"atom": one repeatable unit of user input, as a resolved (post-mapping)
keysequence plus structured fields. Only user actions, not `:normal`,
API calls, or non-"t" `feedkeys`.

BREAKING: dot-repeat of an Insert session, replays the entire session
including cursor-moves (:help ins-repeat).

BREAKING: dot-repeat of a Visual operation, replays the selection
instead of operating on a fixed-size region.

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`.

NEW BUILD SYSTEM!

This is a MVP implementation which supports building the "nvim" binary,
including cross-compilation for some targets.
As an example, you can build a aarch64-macos binary from
an x86-64-linux-gnu host, or vice versa

Add CI target for build.zig currently for functionaltests on linux
x86_64 only

Follow up items:

-  praxis for version and dependency bumping
-  windows 💀
-  full integration of libintl and gettext (or a desicion not to)
-  update help and API metadata files
-  installation into a $PREFIX
-  more tests and linters

**Problem:** Some weird languages have different comment syntax
depending on the location in the code, and we do not have a way to
determine the correct `commentstring` for these special cases.

**Solution:** Allow queries to specify `commentstring` values in
metadata, allowing users/`nvim-treesitter` to provide a better
commenting experience without hugely increasing the scope of the code in
core.

Problem: Currently comment detection, addition, and removal are done
  by matching 'commentstring' exactly. This has the downside when users
  want to add comment markers with space (like with `-- %s`
  commentstring) but also be able to uncomment lines that do not contain
  space (like `--aaa`).

Solution: Use the following approach:
  - Line is commented if it matches 'commentstring' with trimmed parts.
  - Adding comment is 100% relying on 'commentstring' parts (as is now).
  - Removing comment is first trying exact 'commentstring' parts with
    fallback on trying its trimmed parts.

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.