Link children to their parents in AST

[TwitchBronBron]

Several fixes:

• link every child expression/statement to its parent at the beginning of the onFileValidate event. This highly simplifies language service based queries that need to walk up the expression chain to find parent expressions or statements
• use AST walk for BrsFile validation instead of references
• add getSymbolTable() to every AST node to simplify finding the correct symbols.