refactor: remove ultra magical __getitem__ support

[cpcloud]

I'd like to propose the removal of filtering and projection support from __getitem__ and retain only the ability to select single columns from a table expression. __getitem__ functionality from column expressions would be unchanged

The rationale is that the behavior of these methods is hard to predict without semantic understanding of the expressions involved in the call.

Yyou cannot know whether the resulting expression is a projection or a filter without knowing details about the expressions involved.

Here's a simple example of this:

t = ... # table expression
t[t.a]  # is this a projection or filter?
t[[t.a]] # is this the same as ☝🏻?

In contrast, when you see a call to the select method and a call to filter method you know what the resulting expression is.

This would need to go through a deprecation cycle with a few releases before removing it.

[cpcloud]

Going to redo this as a poll.

[cpcloud]

#7259