Enforce conditional values are booleans

Feature

A rule enforcing the strict use of boolean expressions in an if statement. It should be added to the "Miscellaneous strictness flags".

strict_conditional

    TYPE:
        boolean

    DEFAULT:
        False

    Enables or disables strict conditional checks. If True, `mypy` only allows boolean expressions in conditional expressions.

Pitch

I've encountered many hard-to-trace bugs due to non-boolean expressions used in conditionals. For instance, lists, containers, objects, or some literals being checked directly.

if [1,2,3]: # list is not a boolean value, will be treated as False if list is empty or None
    ...

if "Hello world": # str is not a boolean value, will be treated as False if string is empty or None
    ...

if None: # not a boolean, will be treated as False if None
    ...
    
if 2: # not a boolean, might be treated as false if it equals 0
    ...

Very often, the developer's intention was to perform a null-check, or sometimes, an emptiness check, but this syntax yields potential edge cases where the developer did not intend the conditional to yield False.

With strict_conditional enabled, all of the above scenarios should be flagged.

The below examples naturally should always be valid as the expression yields a boolean value.

if x < 0:
    ...

if isinstance(x, int):
    ...

if True:
    ...

if [1,2,3] is None:
    ....

if len({}) == 0:
    ....

This enforces explicit coding practices and better shows the intent of the developer.

Start by locating mypy's conditional-expression checking and the configuration handling for the "Miscellaneous strictness flags". Add the proposed strict_conditional option with a False default, enforce boolean-only conditional expressions when enabled, and cover the listed valid and invalid cases with tests.