[clang-format] Allow excluding preproccesor directives from `IntegerLiteralSeparator`

Given the config

```yaml
IntegerLiteralSeparator:
Decimal: 3
DecimalMinDigits: 5
```

all numbers, *including numbers inside of preprocessor directives*, will have separators inserted:

```cpp
#if __cpp_constexpr >= 201'603L // i.e 2016-03
```

I believe that there is a legitimate use-case for wanting directives to be left as-is, e.g. for feature testing (like above) or if someone wanted to use a C99 preprocessor for their C23/C++14 code.

---

I was thinking the option for it could look something like this:

```yaml
IntegerLiteralSeparator:
Decimal: 3
DecimalMinDigits: 5
PPDirectives: true
```

One could also consider something more fine-grained like `Always`/`Never`/`Leave`, although I can't imagine that being very beneficial.

Start by tracing clang-format's IntegerLiteralSeparator handling for the shown YAML configuration and how integer literals in #if directives are processed. Define and document the intended directive behavior, then add coverage using the configuration and preprocessor example from the issue so the expected unchanged or separated output is explicit.

Written by the indexing model from the issue text.