stdin is a macro in Windows builds.

Some programs behave differently when they detect that stdin is being
piped. This can be problematic when these programs are used with the job
control API where stdin is attached, but not typically used. It is
possible to run the job using a PTY which circumvents this problem, but
that includes a lot of overhead when simply closing the stdin pipe would
suffice.

To enable this behavior, add a new parameter to the jobstart options
dict called "stdin" with two valid values: "pipe" (the default)
implements the existing behavior of opening a channel for stdin and
"null" which disconnects stdin (or, if you prefer, connects it to
/dev/null). This is extensible so that other modes can be added in the
future.

Problem:    Can modify a:000 when using a reference.
Solution:   Make check for locked variable stricter. (Ozaki Kiichi,
            closes vim/vim#3930)
https://github.com/vim/vim/commit/05c00c038bc16e862e17f9e5c8d5a72af6cf7788

Problem:    Cannot use a lambda with quickfix functions.
Solution:   Add support for lambda. (Yegappan Lakshmanan, closes vim/vim#6499)
https://github.com/vim/vim/commit/d43906d2e5969288f239df851f5ad7b1dc2c7251

Problem:    Code to handle callbacks is duplicated.
Solution:   Add callback_T and functions to deal with it.
https://github.com/vim/vim/commit/3a97bb3f0f8bd118ae23f1c97e55d84ff42eef20

Port Vim's put_callback() as callback_put()
because Neovim's naming convention is {type}_{action},
not {action}_{type}.

Renaming put_callback type as PutCallback.
https://neovim.io/develop/style-guide.xml#Type_Names

Problem:    Cannot use "z=" when 'spell' is off.
Solution:   Make "z=" work even when 'spell' is off. (Christian Brabandt,
            Gary Johnson, closes vim/vim#6227)
https://github.com/vim/vim/commit/152e79e94bb935e75b866bd55479648cde11066a

Resolve pvs/v560 error.
mods is nonnull in the modified inner if-condition.
because it is checked for NULL in the outer if-condition.

vim_strsave() returns nonnull.

Windows handles environment variable names case-insensitively (i.e.,
%Path% can be accessed via %PATH%).  Since nvim explicitly creates the
environment block for jobs, we need to ensure it doesn't contain
duplicate names with different cases (e.g., both %Path% and %PATH%) as
that can confuse processes using the environment.

`f_environ()` now converts all env var names to upper-case on Windows,
since it does duplicate detection and also provides the base environment
for jobs.  Similarly, `create_environment()` converts any env var names
supplied in the job opts to upper-case so they can be compared against
what we get from `f_environ()`.

Closes #14541