Python: Add TypeTrackingNode

Splits `ModuleVariableNode` away from `LocalSourceNode`, instead
creating a class `TypeTrackingNode` that encapsulates both of these.

This means we no longer have module variable nodes as part of
`LocalSourceNode` (which is good, since they have no "local" aspect to
them), and hence we can have `LocalSourceNode` inherit directly from
`ExprNode` (which makes the API a bit nicer).

Unfortunately these are breaking changes, so we can't actually fulfil
the above two desiderata until the `track` and `backtrack` methods on
`LocalSourceNode` have been fully deprecated. For this reason, we
preserve the present implementation of `LocalSourceNode`, and instead
lay the foundation for switching over in the future, by deprecating
`track` and `backtrack` on `LocalSourceNode`.

python/.vscode/ql.code-snippets

@@ -106,7 +106,7 @@
         "prefix": "type tracking",
         "body": [
             "/** Gets a reference to ${3:a thing}. */",
-            "private DataFlow::LocalSourceNode ${1:myType}(DataFlow::TypeTracker t) {",
+            "private DataFlow::TypeTrackingNode ${1:myType}(DataFlow::TypeTracker t) {",
             "  t.start() and",
             "  result = ${2:value}",
             "  or",