Refactor: Move compiler node definitions to src/ast/nodes

The `bound_nodes.rs` file has been removed and its contents have been
moved to `src/ast/nodes.rs`. This consolidates all AST node definitions
into a single module, improving organization and maintainability.

The `compiler` modules now import these definitions from
`crate::ast::nodes` instead of `crate::ast::compiler::bound_nodes`.

src/ast/compiler/dumper.rs

@@ -1,4 +1,4 @@
-use crate::ast::compiler::bound_nodes::{CompilerPhase, Node, NodeKind};
+use crate::ast::nodes::{CompilerPhase, Node, NodeKind};
 
 /// Human-readable AST dumper for the bound AST.
 pub struct Dumper {