improve error message when passing a non-identifier to start a connection

[edublancas]

we allow users to pass a connection variable to connect to a db. example:

%load_ext sql

import duckdb
conn = duckdb.connect()

%sql conn

however, in some cases, the connection object might be part of a dictionary, list or object. examples:

%sql object.property
%sql dictionary["key"]
%sql some_list[0]

Trying any of the above throws an error because our parser thinks it's SQL code. Instead, we should provide a more informative error:

"object.property" is not a valid connection identifier. Please pass the variable's name directly, as passing object attributes, dictionaries `some_dict[key]` or lists `some_list[0]` won't work. Example: `%sql connection`

so we need a way to identity such cases, we can use the following heuristics:

• the magic gets a single argument (no spaces between characters) - because valid SQL code will always have spaces
• we can check if the argument is not a valid identifier using argument.isidentifier() but it is valid Python code (using ast.parse)

if those conditions hold, then we can raise the error.

[bbeat2782]

Acceptance Criteria

1. Raise a more informative error when a connection object that is a part of object, dictionary, or list (eg. %sql object.property, %sql dictionary["key"], %sql some_list[0]) is passed
   a. error example :

"object.property" is not a valid connection identifier. Please pass the variable's name directly, as passing object attributes, dictionaries `some_dict[key]` or lists `some_list[0]` won't work. Example: `%sql connection`

2. Add test functions