[ty] AST garbage collection (#18482)

## Summary

Garbage collect ASTs once we are done checking a given file. Queries
with a cross-file dependency on the AST will reparse the file on demand.
This reduces ty's peak memory usage by ~20-30%.

The primary change of this PR is adding a `node_index` field to every
AST node, that is assigned by the parser. `ParsedModule` can use this to
create a flat index of AST nodes any time the file is parsed (or
reparsed). This allows `AstNodeRef` to simply index into the current
instance of the `ParsedModule`, instead of storing a pointer directly.

The indices are somewhat hackily (using an atomic integer) assigned by
the `parsed_module` query instead of by the parser directly. Assigning
the indices in source-order in the (recursive) parser turns out to be
difficult, and collecting the nodes during semantic indexing is
impossible as `SemanticIndex` does not hold onto a specific
`ParsedModuleRef`, which the pointers in the flat AST are tied to. This
means that we have to do an extra AST traversal to assign and collect
the nodes into a flat index, but the small performance impact (~3% on
cold runs) seems worth it for the memory savings.

Part of https://github.com/astral-sh/ty/issues/214.

crates/ruff_python_parser/src/semantic_errors.rs

@@ -118,7 +118,11 @@ impl SemanticSyntaxChecker {
                 // _ = *(p + q)
                 Self::invalid_star_expression(value, ctx);
             }
-            Stmt::Return(ast::StmtReturn { value, range }) => {
+            Stmt::Return(ast::StmtReturn {
+                value,
+                range,
+                node_index: _,
+            }) => {
                 if let Some(value) = value {
                     // test_err single_star_return
                     // def f(): return *x
@@ -638,6 +642,7 @@ impl SemanticSyntaxChecker {
                 range,
                 id,
                 ctx: expr_ctx,
+                node_index: _,
             }) => {
                 // test_err write_to_debug_expr
                 // del __debug__