dillon1000/react

Track next scope id on Environment

Currently we allocate all reactive scopes during a single pass, 
InferReactiveScopeVariables, using a local incrementing number to assign 
ScopeIds. This means we can't easily create additional scopes later since we 
don't know the next available scope id. 

Here we add `Environment.nextScopeId` and use that to synthesize scope ids.