Better message for truthy functions

[madt2709]

Fixes case 1 in #14529.

If this approach is correct, I will add the rest of the cases.

Updated message + tests.

Recommend we squash the commits when we merge since they are not clean but change is small enough to be one commit.

EDIT: This PR doesn't work quite as intended with walrus operator. It outputs two error msgs for both the function value and target. This seemed the least bad solution as:

• the error message is still more informative than before
• requires the least amount of code changes

[hauntsaninja]

Overall looks great! Just had a minor comment

[madt2709]

Thanks for the review @hauntsaninja!

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

arviz (https://github.com/arviz-devs/arviz)
- arviz/tests/base_tests/test_utils.py:250: error: Function "Callable[[Any, Any], Any]" could always be true in boolean context  [truthy-function]
+ arviz/tests/base_tests/test_utils.py:250: error: Function "_stack" could always be true in boolean context  [truthy-function]

spark (https://github.com/apache/spark)
- python/pyspark/ml/torch/distributor.py:658: error: Function "Callable[..., Any]" could always be true in boolean context  [truthy-function]
+ python/pyspark/ml/torch/distributor.py:658: error: Function "framework_wrapper_fn" could always be true in boolean context  [truthy-function]

prefect (https://github.com/PrefectHQ/prefect)
- src/prefect/settings.py:1708: error: Function "Callable[[Settings, T], T]" could always be true in boolean context  [truthy-function]
+ src/prefect/settings.py:1708: error: Function "value_callback" could always be true in boolean context  [truthy-function]
- src/prefect/results.py:101: error: Function "Callable[[TaskRunContext, dict[str, Any]], str | None]" could always be true in boolean context  [truthy-function]
+ src/prefect/results.py:101: error: Function "cache_key_fn" could always be true in boolean context  [truthy-function]
- src/prefect/engine.py:1521: error: Function "Callable[[TaskRunContext, dict[str, Any]], str | None]" could always be true in boolean context  [truthy-function]
+ src/prefect/engine.py:1521: error: Function "cache_key_fn" could always be true in boolean context  [truthy-function]

ibis (https://github.com/ibis-project/ibis)
- ibis/common/caching.py:100: error: Function "Callable[[Any], Any]" could always be true in boolean context  [truthy-function]
+ ibis/common/caching.py:100: error: Function "key" could always be true in boolean context  [truthy-function]

pyppeteer (https://github.com/pyppeteer/pyppeteer)
- pyppeteer/browser.py:54: error: Function "Callable[[], Awaitable[None]]" could always be true in boolean context  [truthy-function]
+ pyppeteer/browser.py:54: error: Function "closeCallback" could always be true in boolean context  [truthy-function]

[hauntsaninja]

Looks great, thank you!