fix: NotNil fails on non-nillable types

[happysnaker]

Summary

Fixes #570

Currently NotNil(t, 0), NotNil(t, ""), NotNil(t, mystruct{}) all pass without error because isNil returns false for non-nillable types, making NotNil always succeed for values like int, string, and struct.

Changes

1. Added isNillableKind helper that checks if a reflect.Kind can hold a nil value
2. Updated NotNil to fail when called with a non-nillable type
3. Refactored isNil to use the new isNillableKind helper

Before

assert.NotNil(t, 0)     // passes (incorrectly)
assert.NotNil(t, "")   // passes (incorrectly)
assert.NotNil(t, MyStruct{}) // passes (incorrectly)

After

assert.NotNil(t, 0)     // fails: Expected value not to be nil, but got non-nillable type int
assert.NotNil(t, "")   // fails: Expected value not to be nil, but got non-nillable type string

[happysnaker]

Tests fail because existing tests use NotNil with non-nillable types like true, 0, "true". This is a breaking change that requires updating those tests. Closing for now — will re-approach with a less breaking fix.