when `terminal.detect_venv.activate_script` setting is default, pick the
appropriate activate script as per the `terminal.shell` settings
specified by the user. Previously when the activate_script setting is
default, zed always try to use the `activate` script, which only works
when the user shell is `bash or zsh`. But what if the user is using
`fish` shell in zed?

Release Notes:

- python: value of `activate_script` setting is now automatically
inferred based on the kind of shell the user is running with.

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Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>