Add `hard_tabs: false` in project settings (#17357)

# Problem

I have a custom system-wide rustfmt configuration, and use tabs over
spaces. So when I contribute to Zed, I will get lots of formatting
errors.

# Proposition

- ~~Add rustfmt.toml (to specify that you are using the default rustfmt
configuration, see https://github.com/rust-lang/cargo/issues/14442)~~
- Add `hard_tabs: false` to `.zed/settings.json` for people using tabs
over spaces.

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>