feat: add type aliases in local scope

[Wodann]

Issue #110 introduced type aliases to Mun - on a file-level. We want to extend this, such that - similar to Rust - Mun can support type aliases in a local scope:

type OtherTypeName = i32;
fn main() {
    type Typename = OtherTypeName; // Type alias in local scope
    let a: Typename = 10;
}

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• claim this issue (assign yourself or comment below)
• implement the feature and start a Pull Request with in the description: closes feat: add type aliases in local scope #265.

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