Fix query builder expressions

[circulon]

This PR addresses several issues:

• case-sensitivity in expression builder classes (alias construction and order direction identifiers)
• query construction in the QueryBuilder when using table/column aliases

example:

builder = QueryBuilder().table("auth_user AS au").select("email")
print(builder.to_sql())
>>> SELECT "auth_user AS au"."email" FROM "auth_user AS au"

Issues addressed

• Fixed in expression builder classes
  • FromTable not handling "as " at all producing invalid syntax.
  • the "AS" keyword had to be lowercase or it was not recognised and split correctly
  • the "ASC" and "DESC" keywords had to be lowercase or it was not recognised correctly
  • removed redundant alias parameter from AggregateExpression as its now handled correctly by the column helper
• Fixed in QueryBuilder
  • added raw parameter to from_ to align with the table method
  • added a deprecation warning to aggragate() about not using the alias parameter
• General
  • removed redundant process_table() from MySqlGrammar
  • alias identifiers are now not quoted as this is the accepted standard in sql syntax
    • if an alias is a keyword or contains special chars then raw=True can be used
  • renamed postgres grammar test files to align with naming of other dialects files
  • added tests