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`.
Replacing `Value.booleanValue`. We wanted to match `Object.booleanValue` that
only gives a result if it is either `true` or `false`, but also wanted to keep
the flexibility to see if the Value _could_ be `true`/`false`. We don't have a
motivating usecase, so let's see if we ever need it :P
+ fix modernisation regression on py/jinja2/autoescape-false
