starlark: change type(len) to "builtin_function_or_method"

Before, any Callable had a type of 'function', but this was a mistake.

The Callable interface itself is now described as "callable"
(e.g. in type error messages), but implementations must define their
own type string. Bazel now uses "rule" and "repository_rule" for its two.

This is technically a breaking change, though I expect the fallout to be small.
(I found no problematic uses in Google's code base.)

PiperOrigin-RevId: 339098833