macOS terminal: Option can't type @, #, |, [ ], { } on many keyboard layouts

[jcxplorer]

On macOS, the integrated terminal treats Option as Alt. On many keyboard layouts, Option is how you type basic characters: Option+L for @ on German, Option+7 for | on Finnish and Swedish, Option+3 for # on British, and Option+8/9 for [ ] on several others. In the terminal, those key presses send an Alt shortcut, so typing an email address or a shell pipe doesn't work (#13046). Several fixes have been attempted, but none were merged.

Fixing this properly needs a setting. Some people rely on Option as Alt for shell shortcuts like Option+B and Option+F, and those shortcuts break if Option always types characters. Other terminals handle this as follows:

• Terminal.app and iTerm2 type the character by default, and have a setting to use Option as Meta.
• Ghostty has macos-option-as-alt (off / left / right / both) and picks its default from the keyboard layout.

I wouldn't copy Ghostty's automatic default, because the browser doesn't reliably tell us what the layout puts on Option.

I opened #19833 with this approach, and it was closed because these choices need maintainer agreement first. Before I open a new PR, I'd like to agree on a few things:

1. The default. I'd suggest typing the character, like Terminal.app and iTerm2. Users on these layouts currently can't type basic characters at all, while Alt users lose shortcuts they can turn back on. The alternative is to keep Alt as the default and have affected users find the setting.
2. The setting. I'd suggest a per-device "Option as Alt in terminal" setting with Off / Left Option / Right Option / Both, like Ghostty. Being able to choose a side lets people keep one Option key for characters and use the other as Alt. If a simple on/off is preferred, I can do that instead.
3. Where it lives. fix(web): macOS Option types layout characters in the terminal #19833 put it in General → Behavior. A small "Terminal" section in General might be easier to find, and would give future terminal settings a home. It would only hold this setting for now, since Terminal font fits better in Appearance → Typography. I'm happy to go either way.

The setting would only appear on macOS. iOS already types these characters natively, and other platforms are unaffected.

I've tested #19833 in Chrome and Safari with a Finnish layout, and I'm happy to adjust it to whatever direction you agree on.

[jcxplorer]

A good first step would be to merge what @neominik did in #21596. It fixes the main problem, and the default matches Terminal.app and iTerm2.

I still think there should be a setting on top of it, so that anyone who actually wants Option to behave as Alt can choose to.