* test: prove bug in cognitive complexity rule
* fix: cognitive complexity nesting with if-else chains
Currently, an if-else chain will increase the nesting level and add the
nesting increment for every addition `else if` statement in an if-else
chain. This is incorrect behaviour; an `else if` statement should
increment complexity by 1 (regardless of current nesting level) and
leave the nesting level as-is.
For example, the following should yield a total complexity of 5:
```
for { // +1
if a { // +2 (nesting = 1)
foo()
} else if b { // +1
bar()
} else if c { // +1
baz()
}
}
```
but the current implementation incorrectly increments the nesting level
with each `else if` and adds the nesting increment where it shouldn't:
```
for { // +1
if a { // +2 (nesting = 1)
foo()
} else if b { // +3 (nesting = 2)
bar()
} else if c { // +4 (nesting = 3)
baz()
}
}
```
