In GUI Vim (gvim, MacVim), each system() call incurs significant
overhead due to command prompt window creation, making operations
like PlugDiff ~5x slower than in terminal Vim or Neovim.
Add s:system_job() which uses job_start with file-based output to
run commands without that overhead, and use it in s:system() when
running in GUI Vim with job support.
Since v:shell_error is read-only in Vim 9, introduce s:shell_error
which is set by both code paths and used at all call sites.
Fixes#1312
