Disable flow control by default

[xiaq]

What new feature should Elvish have?

Flow control seems rarely used these days and disabling it allows us to reclaim Ctrl-Q and Ctrl-S.

Fish already does this by default, and zsh has an option for that.

https://github.com/elves/elvish/issues/1798#issuecomment-2093480971

Output of "elvish -version"

v0.21.1

Code of Conduct

• [X] I agree to follow Elvish's Code of Conduct.

[krader1961]

See also https://github.com/elves/elvish/issues/732. I agree that software flow-control should be disabled by default. Whether it should be possible for a user to enable it using the stty command is debatable but the answer is probably "no". I'm old enough to remember using software flow-control a lot. However, its utility diminished as the speed of the connection between the terminal and computer increased. When the transmission speed was equal to or greater than that of a 28.8K bps connection (e.g., a modem you might buy in 1995) software flow-control became only marginally useful.