Adjust TintColor color token terminology (#22826)

Previously, to use a green and red shade with `TintColor` you'd need to
pass `Positive` and `Negative`, respectively. This terminology always
tripped me up, because, for example, I'd often try to use something
like:

```
Button::new("icon_color", "Negative")
      style(ButtonStyle::Tinted(TintColor::Negative))
      .color(Color::Error)
      .icon_color(Color::Error)
      .icon(IconName::Trash),
)
```

...and due to `icon_color` taking `Color::Error`, I'd always get
`TintColor` wrong at a first try, because I would, out of muscle memory,
write `TintColor::Error`, which wouldn't compile. That's exactly the
change in this PR—`TintColor` now takes `Success` and `Error` instead of
`Positive` and `Negative`, for more consistency.


Release Notes:

- N/A

crates/assistant/src/assistant_panel.rs

@@ -3654,7 +3654,7 @@ impl ContextEditor {
 
         let (style, tooltip) = match token_state(&self.context, cx) {
             Some(TokenState::NoTokensLeft { .. }) => (
-                ButtonStyle::Tinted(TintColor::Negative),
+                ButtonStyle::Tinted(TintColor::Error),
                 Some(Tooltip::text("Token limit reached", cx)),
             ),
             Some(TokenState::HasMoreTokens {
@@ -3711,7 +3711,7 @@ impl ContextEditor {
 
         let (style, tooltip) = match token_state(&self.context, cx) {
             Some(TokenState::NoTokensLeft { .. }) => (
-                ButtonStyle::Tinted(TintColor::Negative),
+                ButtonStyle::Tinted(TintColor::Error),
                 Some(Tooltip::text("Token limit reached", cx)),
             ),
             Some(TokenState::HasMoreTokens {