fix(exepath)!: prefers extensionless for powershell

PROBLEM:
exepath("test") should prefer ".bat" when shell=powershell.
Current behavior differs from Vim 8.2.3071.

TEST CASE:
1. in a folder which is in $PATH, create files "test" "test.bat".
   - "(Get-Command test).Path test" returns "test.bat".
2. compare nvim:
     nvim --clean
     :set shell=powershell
     :echo exepath("test")
3. should returns "path\to\test.bat" (before this patch it returns "path\to\test").

SOLUTION:
After this patch, the binary files "text.exe", "test.bat", "test.com"
will be found, but the file "test" (without any extension) will not be
found (matches Vim 8.2.3071). But powershell's `where` and
`Get-Command` _do_ find the extensionless 'test' file.

But Nvim with ":set shell=cmd.exe", doesn't find "test" either (before
and after this patch), even though `where test` returns correct path in
cmd.

- `where` is a program to find files in general, not just executable files.
-`Get-Command` returning extensionless (and thus non-executable) file is
  puzzling even to Chris Dent, [PowerShell expert][1] (asked on
  Discord).

[1]: https://www.amazon.com/Mastering-PowerShell-Scripting-Automate-environment-ebook/dp/B0971MG88X

Co-authored-by: Enan Ajmain <3nan.ajmain@gmail.com>
Helped-by: erw7 <erw7.github@gmail.com>

Fixes #21045

src/nvim/os/fs.c

@@ -302,7 +302,9 @@ static bool is_executable(const char *name, char **abspath)
 static bool is_executable_ext(const char *name, char **abspath)
   FUNC_ATTR_NONNULL_ARG(1)
 {
-  const bool is_unix_shell = strstr((char *)path_tail(p_sh), "sh") != NULL;
+  const bool is_unix_shell = strstr(path_tail(p_sh), "powershell") == NULL
+                             && strstr(path_tail(p_sh), "pwsh") == NULL
+                             && strstr(path_tail(p_sh), "sh") != NULL;
   char *nameext = strrchr(name, '.');
   size_t nameext_len = nameext ? strlen(nameext) : 0;
   xstrlcpy(os_buf, name, sizeof(os_buf));