BraceStyle AlwaysNewLine for extern blocks

[blesson3]

• extern blocks can now get an opening brace on a new line

• add test case for extern blocks

I also made sure that there are no trailing spaces for the extern "C" line after formatted.

fixes issue #4428

[blesson3]

Example extern formatting for brace_style: AlwaysNextLine:

Before formatting (source):

extern "Rust" {
    fn foo(x: i32);
}

extern "cdecl" {}

extern "fastcall" {/* f*/}

After formatting (target):

extern "Rust" 
{
    fn foo(x: i32);
}

extern "cdecl" {}

extern "fastcall"
{
    /* f*/
}

[calebcartwright]

Thanks!