Style: Harmonize header includes in platform ports

This applies our existing style guide, and adds a new rule to that style
guide for modular components such as platform ports and modules:

Includes from the platform port or module should be included with relative
paths (relative to the root folder of the modular component, e.g.
`platform/linuxbsd/`), in their own section before Godot's "core" includes.

The `api` and `export` subfolders also need to be handled as self-contained
(and thus use relative paths for their "local" includes) as they are all
compiled for each editor platform, without necessarily having the api/export
matching platform folder in the include path.
E.g. the Linux editor build will compile `platform/android/{api,export}/*.cpp`
and those need to use relative includes for it to work.

platform/web/web_main.cpp

@@ -28,17 +28,17 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                 */
 /**************************************************************************/
 
+#include "display_server_web.h"
+#include "godot_js.h"
+#include "os_web.h"
+
 #include "core/config/engine.h"
 #include "core/io/resource_loader.h"
 #include "main/main.h"
-#include "platform/web/display_server_web.h"
-#include "platform/web/os_web.h"
 
 #include <emscripten/emscripten.h>
 #include <stdlib.h>
 
-#include "godot_js.h"
-
 static OS_Web *os = nullptr;
 static uint64_t target_ticks = 0;
 static bool main_started = false;