Bad support for subqueries in the FROM clause

[filipsch]

See this example on ast-viewer.datacamp.com.

A bunch of unshaped nodes.

[machow]

It looks like quite a few things can go into the FROM clause in a SELECT statement. If we just want to add support for subqueries here, I think we can just handle that case in the dml_expression_clause rule in the grammar.

https://github.com/datacamp/antlr-plsql/blob/master/antlr_plsql/plsql.g4#L1293

From what I can tell the three changes are:

1. support a list of table expressions in the FROM clause (see this query)
2. implement some AST structure for table aliases (see this query)
3. handle subqueries in the FROM clause (see this query)