Add AST dumper and improve upvalue analysis

The `UpvalueAnalyzer` now correctly identifies which lambdas capture
which variable declarations, rather than just marking declarations that
need boxing. This information is stored in a `capture_map`.

The `Binder` has been updated to use this new `capture_map` instead of a
`HashSet` of boxed declarations. The `DefLocal` node in `bound_nodes.rs`
now stores a `captured_by` field, which is a list of lambda identities
that capture the local variable.

A new `dumper` module has been added to provide a human-readable
representation of the bound AST, including information about captured
variables.

The `VM` has been updated to use the `captured_by` field to determine if
a local variable needs to be stored in a `Cell`, rather than relying on
a boolean `is_boxed` flag.

An example script `extreme_capture.myc` has been added to test deep
nesting and variable capture.

A new "Dump AST" button has been added to the UI, which uses the new
`Dumper` to display the bound AST.

src/ast/compiler/bound_nodes.rs

@@ -1,5 +1,5 @@
 use std::rc::Rc;
-use crate::ast::types::{Value, StaticType};
+use crate::ast::types::{Value, StaticType, Identity};
 use crate::ast::nodes::Node;
 
 #[derive(Debug, Clone, Copy, PartialEq)]
@@ -27,7 +27,7 @@ pub enum BoundKind {
     DefLocal {
         slot: u32,
         value: Box<Node<BoundKind, StaticType>>,
-        is_boxed: bool,
+        captured_by: Vec<Identity>,
     },
 
     If {