feat: Add support for function declarations.

src/ast.rs

@@ -103,3 +103,21 @@ pub enum Type {
 
     Named { name: Box<str>, name_span: Span },
 }
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum Declaration {
+    Function {
+        name: Box<str>,
+        name_span: Span,
+        parameters: Vec<Parameter>,
+        return_type: Option<Type>,
+        body: Option<Statement>,
+    },
+}
+
+#[derive(Debug, PartialEq, Eq)]
+pub struct Parameter {
+    pub name: Box<str>,
+    pub name_span: Span,
+    pub type_: Type,
+}