feat: Add AST emitter and comment support

Introduces a new AST emitter module responsible for serializing AST
nodes back into Myc source code. This emitter preserves leading comments
and whitespace, enabling a round-trip guarantee for the parser and
emitter.

The documentation in `docs/BNF.md` has been updated to reflect the new
syntax rules for comments, including single-line comments (`;`) and
documentation comments (`;;`). The rules clarify that comments must be
whole lines and that inline comments are disallowed.

The lexer and AST node structures have been updated to accommodate
`CommentLine` variants, allowing for different types of comments
(regular, documentation, and blank lines) to be stored and processed.

A new test suite `tests/comment_roundtrip.rs` has been added to verify
the round-trip property of the parser and emitter across all example
`.myc` files. This test ensures that parsing source code, emitting it,
and then parsing the emitted code again results in the same output
string.

The `ast.rs` binary now includes a `--emit` flag that uses the new
emitter to output the parsed source code, facilitating debugging and
testing of the emitter itself.

src/ast/compiler/macros.rs

@@ -624,7 +624,7 @@ mod tests {
     use crate::ast::diagnostics::Diagnostics;
     use crate::ast::nodes::{Address, VirtualId};
     use crate::ast::parser::Parser;
-    use crate::ast::types::{NodeIdentity, Object, Purity, SourceLocation, StaticType, Value};
+    use crate::ast::types::{NodeIdentity, Purity, SourceLocation, StaticType, Value};
 
     struct SimpleEvaluator;
     impl MacroEvaluator for SimpleEvaluator {