feat(notgraph): implement symbols extractor

crates/notgraph/tests/fixtures/symbols/src/lib.rs

@@ -0,0 +1,7 @@
+pub struct Foo;
+pub enum Bar { A, B }
+pub trait Baz { fn run(&self); }
+pub fn qux() {}
+pub type Alias = u32;
+pub const VALUE: u32 = 42;
+struct Private;