Support configurable terminal scroll speed/scroll multiplier

[ethanal]

There are a number of issues open related to scroll speed in the terminal on macOS:

• Scroll speed in tmux in terminal emulator is way too fast #18930
• Scrolling in the terminal feels yucky #10425

In my case, the terminal scroll speed feels way too fast compared to the editor.

On macOS, there is a hard-coded scroll multiplier for the terminal here:

///Scrolling is unbearably sluggish by default. Alacritty supports a configurable
///Scroll multiplier that is set to 3 by default. This will be removed when I
///Implement scroll bars.
#[cfg(target_os = "macos")]
const SCROLL_MULTIPLIER: f32 = 4.;
#[cfg(not(target_os = "macos"))]
const SCROLL_MULTIPLIER: f32 = 1.;

On macOS, I tried setting this scroll multiplier to 1.0 and rebuilding, and I find the scroll much easier to use this way.

Interestingly, this older issue suggests the opposite problem and appears to be opened after the introduction of the scroll speed multiplier:

• Scrolling in terminal tab is slower than in editor tab #5130

So maybe the takeaway is that there's a lot of personal preference in scroll speed, and perhaps users have embedded terminal multiplexers or some other situation, so there's reason to customize the scroll multiplier/sensitivity.

I'm proposing a new setting, like the following, which would take the place of the SCROLL_MULTIPLIER constant:

"terminal": {
  "scroll_sensitivity": 1.0
}

For comparison, VSCode supports customizable scroll sensitivity for both the editor and the terminal.

In terms of defaults for this setting -- I'm unconvinced that the current default behavior on macOS is what most users would want, since it differs significantly from most macOS terminal scroll behavior I've seen.

[daliborfilus]

If this isn't a separate configurable item, I would hope it is at least set to the same as the editor scroll_sensitivity.
Please, make the sensitivity affect scrolling everywhere in Zed (editor, project, terminal), or make separate scrolling settings for each of them.

Example: I use Logitech G603.
The editor scrolls the same speed as, e.g. Apple Mail app. So the scrolling is too fast in all apps, that's not Zed's fault.
But the terminal scrolls faster.

Setting scroll_sensitivity to 0.25 is better for the mouse (and terribly slow for the trackpad), but it affects just the editor. It's not "the fix" for the terminal, though.

(I also use SteerMouse with a default scrolling curve - if I disable it, it scroll behavior is the same, so it matches the Apple's scrolling behavior.)

EDIT: Further testing reveals that if I override "scroll: 1 line" in SteerMouse, with scroll_sensitivity: 1.0, the Apple Mail scrolls by 1 line, Zed's project and editor views scroll by 1 line, but the terminal scrolls by 4 lines. Why?