Automatically adjust ANSI color contrast (#34033)

Closes #33253 in a way that doesn't regress #32175 - namely,
automatically adjusts the contrast between the foreground and background
text in the terminal such that it's above a certain threshold. The
threshold is configurable in settings, and can be set to 0 to turn off
this feature and use exactly the colors the theme specifies even if they
are illegible.

## One Light Theme Before
<img width="220" alt="Screenshot 2025-07-07 at 6 00 47 PM"
src="https://github.com/user-attachments/assets/096754a6-f79f-4fea-a86e-cb7b8ff45d60"
/>

(Last row is highlighted because otherwise the text is unreadable; the
foreground and background are the same color.)

## One Light Theme After

(This is with the new default contrast adjustment setting.)

<img width="215" alt="Screenshot 2025-07-07 at 6 22 02 PM"
src="https://github.com/user-attachments/assets/b082fefe-76f5-4231-b704-ff387983a3cb"
/>

This approach was inspired by @mitchellh's use of automatic contrast
adjustment in [Ghostty](https://ghostty.org/) - thanks, Mitchell! The
main difference is that we're using APCA's formula instead of WCAG for
[these
reasons](https://khan-tw.medium.com/wcag2-are-you-still-using-it-ui-contrast-visibility-standard-readability-contrast-f34eb73e89ee).

Release Notes:

- Added automatic dynamic contrast adjustment for terminal foreground
and background colors

crates/repl/src/outputs/plain.rs

@@ -25,6 +25,7 @@ use alacritty_terminal::{
 use gpui::{Bounds, ClipboardItem, Entity, FontStyle, TextStyle, WhiteSpace, canvas, size};
 use language::Buffer;
 use settings::Settings as _;
+use terminal::terminal_settings::TerminalSettings;
 use terminal_view::terminal_element::TerminalElement;
 use theme::ThemeSettings;
 use ui::{IntoElement, prelude::*};
@@ -257,8 +258,17 @@ impl Render for TerminalOutput {
                 point: ic.point,
                 cell: ic.cell.clone(),
             });
-        let (cells, rects) =
-            TerminalElement::layout_grid(grid, 0, &text_style, text_system, None, window, cx);
+        let minimum_contrast = TerminalSettings::get_global(cx).minimum_contrast;
+        let (cells, rects) = TerminalElement::layout_grid(
+            grid,
+            0,
+            &text_style,
+            text_system,
+            None,
+            minimum_contrast,
+            window,
+            cx,
+        );
 
         // lines are 0-indexed, so we must add 1 to get the number of lines
         let text_line_height = text_style.line_height_in_pixels(window.rem_size());