variable_name rule should extend to all identifiers

[koke]

The Swift API Design Guidelines state that:

Names of types and protocols are UpperCamelCase. Everything else is
lowerCamelCase.

The existing variable_name rule only checks that variables are lowerCamelCase, but other identifiers, like functions or methods aren't checked:

// No error
func Function(param: Int) -> Int {
    return param
}

// error: Variable Name Violation: Variable name should start with a lowercase character: 'Function' (variable_name)
let Function: Int -> Int = { $0 }

I'd propose renaming variable_name to identifier_name and check all the other identifier types as well.

Related: #660

[ColesyM]

How do we disable the lower case check in config, for now I have had to disable this entire rule :(