fix: correctly identify powershell

[mdonnalley]

Correctly distinguish between powershell and cmd.exe

@W-11446819@

[cristiand391]

left one suggestion to improve detection on linux/mac

[cristiand391]

QA notes:

setup:

linked this branch of oclif into plugin-autocomplete
win: patched oclif/core in sf install folder

general:
✅ SHELL still takes precedence. This is usually set at the system-level, can be modified in terminals/multiplexers when needed

mac:
🟡 powershell detected:
because SHELL is set by default, the only way to make oclif detect powershell correctly is to make your term/multiplexer unset it/set to SHELL=powershell. So, the current behavior of detecting powershell on mac/linux doesn't change.

win:
✅ powershell detected
SHELL isn't set and os.userInfo().shell is always null:
https://nodejs.org/api/os.html#osuserinfooptions

Returns information about the currently effective user. On POSIX platforms, this is typically a subset of the password file. The returned object includes the username, uid, gid, shell, and homedir. On Windows, the uid and gid fields are -1, and shell is null.

so oclif does the process.title check.
I can run sf autocomplete alone instead of sf autocomplete powershell and get valid pwsh instructions.

here I ran sf autocomplete --dev-debug on windows's cmd to verify config.shell value is still cmd.exe (this is also set by the process.title check b/c the COMPSEC one has lower precedence.