In the `enc=utf-8` only world, the 'isprint' option is quite obsolete.
The function is only consulted for decoded U+0080 to U+00FF chars
whose printability is defined by the unicode charset, just like
higher codepoints. Meanwhile, invalid `\x80` to `\xFF` bytes are always
unprintable.

Disentangle 'iskeyword', 'isfname' and 'isident' from each other by
giving them all separate bitsets. Now these can be updated individually.
These updaters can back out of errors gracefully, no need for extra
panic logic in option[str].c to restore partially updated chartabs.

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`.

Use only a single clear() call in some test/functional/vimscript/ test
files whose test cases have very little side effect.

A downside of using a single clear() is that if a crash happens in one
test case, all following test cases in the same file will also fail, but
these functionalities and tests don't change very often.

In the api_info() output:

    :new|put =map(filter(api_info().functions, '!has_key(v:val,''deprecated_since'')'), 'v:val')
    ...

    {'return_type': 'ArrayOf(Integer, 2)', 'name': 'nvim_win_get_position', 'method': v:true, 'parameters': [['Window', 'window']], 'since': 1}

The `ArrayOf(Integer, 2)` return type didn't break clients when we added
it, which is evidence that clients don't use the `return_type` field,
thus renaming Dictionary => Dict in api_info() is not (in practice)
a breaking change.

This is a breaking change which will make refactor of typval and shada
code a lot easier. In particular, code that would use or check for
v:msgpack_types.binary in the wild would be broken. This appears to be
rarely used in existing plugins.

Also some cases where v:msgpack_type.string would be used to represent a
binary string of "string" type, we use a BLOB instead, which is
vimscripts native type for binary blobs, and already was used for BIN
formats when necessary.

msgpackdump(msgpackparse(data)) no longer preserves the distinction
of BIN and STR strings. This is very common behavior for
language-specific msgpack bindings. Nvim uses msgpack as a tool to
serialize its data. Nvim is not a tool to bit-perfectly manipulate
arbitrary msgpack data out in the wild.

The changed tests should indicate how behavior changes in various edge
cases.

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.