Make terminal.option_as_meta=false in default settings (#19364)

- This reverts the change I made in https://github.com/zed-industries/zed/pull/15535 which set `option_as_meta` to `true` in the default settings.
- `true` is a reasonable default for US Keyboards, but is terrible for many others which rely on `alt+<key>` for totally normal keystroke combinations.