Add emath::Vec2b, replacing egui_plot::AxisBools (#3543)

Thanks to `impl From<bool> for Vec2b` one can now shorten some builder
calls, like:

Previous:
```rust
 egui::ScrollArea::vertical()
        .auto_shrink([false; 2])
```

New:
```rust
 egui::ScrollArea::vertical()
        .auto_shrink(false)
```

crates/emath/src/vec2b.rs

@@ -0,0 +1,60 @@
+/// Two bools, one for each axis (X and Y).
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+pub struct Vec2b {
+    pub x: bool,
+    pub y: bool,
+}
+
+impl Vec2b {
+    pub const FALSE: Self = Self { x: false, y: false };
+    pub const TRUE: Self = Self { x: true, y: true };
+
+    #[inline]
+    pub fn new(x: bool, y: bool) -> Self {
+        Self { x, y }
+    }
+
+    #[inline]
+    pub fn any(&self) -> bool {
+        self.x || self.y
+    }
+}
+
+impl From<bool> for Vec2b {
+    #[inline]
+    fn from(val: bool) -> Self {
+        Vec2b { x: val, y: val }
+    }
+}
+
+impl From<[bool; 2]> for Vec2b {
+    #[inline]
+    fn from([x, y]: [bool; 2]) -> Self {
+        Vec2b { x, y }
+    }
+}
+
+impl std::ops::Index<usize> for Vec2b {
+    type Output = bool;
+
+    #[inline(always)]
+    fn index(&self, index: usize) -> &bool {
+        match index {
+            0 => &self.x,
+            1 => &self.y,
+            _ => panic!("Vec2b index out of bounds: {index}"),
+        }
+    }
+}
+
+impl std::ops::IndexMut<usize> for Vec2b {
+    #[inline(always)]
+    fn index_mut(&mut self, index: usize) -> &mut bool {
+        match index {
+            0 => &mut self.x,
+            1 => &mut self.y,
+            _ => panic!("Vec2b index out of bounds: {index}"),
+        }
+    }
+}