Problem: Tee-Object does not create a file if it does not receive input
for example when :grep does not find matches.
and so nvim tries to open a nonexistent errorfile causing an error.
Solution: use tee.exe instead of Tee-Object
Problem:
On Windows, :make does not display the output of the program it runs.
The cause is the default 'shellpipe'. On Linux, nvim uses `tee` to redirect the
output to both stdout and the error file. In Windows, for both cmd.exe and
powershell, the output is only redirected to the error file.
Solution:
- On Windows, change the 'shellpipe' default to "2>&1| tee".
- Nvim includes `tee` in its Windows package.
- Document recommended defaults for powershell.
Fixes#12910
Ensure that
* Shell uses UTF8 input/output mode
* Stderr output is captured, in UTF8
* Program exit codes are correctly captured
Update functional test harness and add tests
for :make command.
Closes#13713
