Auto-declare variables in reactive declarations, in certain cases

[Rich-Harris]

At present you have to declare all variables that are assigned to in reactive declarations:

let b;
$: b = a + 1;

It would be much nicer to just do this:

$: b = a + 1;

In other words if the body is an ExpressionStatement whose expression is an AssignmentExpression whose left hand side is an Identifier, inject the let b.