Meta: Add a script check the presence of "#pragma once" in header files

.. and make travis run it.

I renamed check-license-headers.sh to check-style.sh and expanded it so
that it now also checks for the presence of "#pragma once" in .h files.

It also checks the presence of a (single) blank line above and below the
"#pragma once" line.

I also added "#pragma once" to all the files that need it: even the ones
we are not check.
I also added/removed blank lines in order to make the script not fail.

I also ran clang-format on the files I modified.

Libraries/LibJS/MarkupGenerator.h

@@ -24,6 +24,8 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#pragma once
+
 #include <AK/HashTable.h>
 #include <AK/String.h>
 #include <LibJS/Forward.h>
@@ -61,4 +63,4 @@ private:
     static String wrap_string_in_style(String source, StyleType type);
 };
 
-}
+}

Libraries/LibJS/Runtime/Date.h

@@ -24,6 +24,8 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#pragma once
+
 #include <LibCore/DateTime.h>
 #include <LibJS/Runtime/Object.h>
 

Libraries/LibJS/Runtime/Exception.h

@@ -24,6 +24,8 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#pragma once
+
 #include <LibJS/Runtime/Cell.h>
 #include <LibJS/Runtime/Value.h>