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/egui_demo_app/src/apps/custom3d_wgpu.rs

@@ -99,7 +99,7 @@ impl eframe::App for Custom3d {
     fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
         egui::CentralPanel::default().show(ctx, |ui| {
             egui::ScrollArea::both()
-                .auto_shrink([false; 2])
+                .auto_shrink(false)
                 .show(ui, |ui| {
                     ui.horizontal(|ui| {
                         ui.spacing_mut().item_spacing.x = 0.0;