Starlark: Disallow conversion of `select` to bool

### Description of the problem / feature request:

The return type of `select` converts silently in truthy contexts to `True`.
This can lead to the following code being legal, but not doing what the programmer intended to do:
```python
def is_linux():
return select({
"@platforms//os:linux": True,
"//conditions:default": False
})

def my_macro()
if is_linux():
# do one thing
else:
# do another thing
```

This can lead to behavior that bazel novices would think of as unexpected (the if always evaluates to the true branch).
Instead, bazel should emit a warning or an error whenever the return value of select gets converted to a bool.
I cannot imagine any valid use case where the return of a select should be converted to a truth value (it's always `True` anyways), so taking this behavior away, while a breaking change, would be an improvement for bazel.
### Feature requests: what underlying problem are you trying to solve with this feature?
Improve the user experience for a more confusing part of starlark (selects), by providing a clear error on a beginner mistake.

### Have you found anything relevant by searching the web?

The documentation [calls this starlark behavior out](https://docs.bazel.build/versions/main/configurable-attributes.html#why-does-select-always-return-true), but I believe that in this case, starlark should diverge from the pythonic way that everything silently converts to bool without fail.
The reasoning for that is mainly that understanding `select` and the bazel/starlark execution semantics is difficult enough as-is, and this change would remove a pitfall.

Begin with the configurable-attributes documentation linked in the issue, then locate the Starlark implementation and tests governing select truth-value conversion. Done means the proposed warning or error is defined, implemented, and covered for conversions of select results to bool without breaking valid behavior.

Written by the indexing model from the issue text.