Problem:
The calculation of `len` in `make_filter_cmd` for powershell falls short
by one character for the following ex command:
  :%w !sort
This command satisfies these conditions:
  - `itmp` is not null
  - `otmp` is null
__NOTE__ that other shells circumvent this bug only because of `len`
allocation for six extra characters: a pair of curly braces and four
spaces:
  https://github.com/neovim/neovim/blob/cfdb4cbada8c65aa57e69776bcc0f7b8b298317a/src/nvim/ex_cmds.c#L1551-L1554
If allocation for these six characters are removed, then bash also faces
the same bug.

Solution:
Add allocation for 6 extra bytes. 1 would do, but let's keep powershell
in sync with other shells as much as possible.

Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.

Problem:
If the shell command passed to the filtered bang command isn't found,
the error isn't redirected to the temp.out file when shell is set to
powershell.

Solution: Use anonymous function with Invoke-Command operator (&).

Problem:
`Start-Process` requires the command to be split into the shell
command and its arguments. Previously it was done by parsing, which
didn't handle cases such as
  - commands with escaped space in their filepath
  - quoted commands with space in their filepath

Solution:
Use
  - `pwsh -Command` instead of `Start-Process`
  - `Get-Content` instead of `-RedirectStandardInput`
  - `Out-File` instead of `-RedirectStandardOutput`

Problem:
Since 0b9664f5240be4d9e9d6882fcd398970fd3a9532 powershell filtered
:[range]! commands with args causes error:
"Start-Process: A positional parameter cannot be found that accepts argument ..."

Solution:
Pass args to Start-Process via `-ArgumentList`.
closes #19250

Problem:
The "calls executable in $PATH" is skipped on some CI jobs because
"python" is not found.

Solution:
Use "python3" instead.

Also:
- Add a describe('shell :!') section to system_spec.
- Make the test for #16271 work on systems without powershell.

Problem:
Nvim fails to create tempfile "…/nvim6UJx04/7" when 'shell' is set to
pwsh (PowerShell Core). This breaks filtered shell commands ":{range}!".
With shell set to cmd, it works.

Solution:
PowerShell doesn't use "<" for stdin redirection. Instead, use
"-RedirectStandardInput".

Closes #15913