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 2024 --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, 1 deletions

diff --git a/engine/Cargo.toml b/engine/Cargo.toml
index 9b91df8..6187bb4 100644
--- a/engine/Cargo.toml
+++ b/engine/Cargo.toml
@@ -1,7 +1,7 @@
 [package]
 name = "engine"
 version = "0.1.0"
-edition = "2024"
+edition = "2021"
 
 [dependencies]
 glutin = "0.32.3"