refactor(engine): use Rust edition 2021

This change is because of a rustfmt bug in the 2024 edition.

For example, the following code snippet:
```
struct Value {}
struct ParsingError {}
trait Keyword {}

impl Value {
    fn parse<KeywordT: Keyword>(value: &str, line_no: usize) -> Result<Self, ParsingError> {
        todo!();
    }
}
```

when formatted with `rustfmt --edition 2021 --emit stdout --config max_width=90,brace_style=AlwaysNextLine` becomes:
```
struct Value {}
struct ParsingError {}
trait Keyword {}

impl Value
{
    fn parse<KeywordT: Keyword>(value: &str, line_no: usize)
    -> Result<Self, ParsingError>
    {
        todo!();
    }
}
```

1 files changed, 1 insertions, 2 deletions

diff --git a/engine/src/windowing/mouse.rs b/engine/src/windowing/mouse.rs
index 2a3324e..f4969ea 100644
--- a/engine/src/windowing/mouse.rs
+++ b/engine/src/windowing/mouse.rs
@@ -1,7 +1,6 @@
 use std::collections::HashMap;
 
 use crate::ecs::Sole;
-
 use crate::vector::Vec2;
 
 #[derive(Debug, Default, Clone, Sole)]
@@ -48,7 +47,7 @@ impl Buttons
 
     /// Returns a iterator that yields buttons and their current states. Only buttons with
     /// states is included.
-    pub fn all_current(&self) -> impl Iterator<Item = (Button, ButtonState)>
+    pub fn all_current(&self) -> impl Iterator<Item = (Button, ButtonState)> + use<'_>
     {
         self.map
             .iter()