wasm: add preload config for shared libraries build

Preload Qt plugins and QML imports by default. The json
files contain file listings generated by scripts in
qtbase/util/wasm/preload.

Task-number: QTBUG-63925
Change-Id: I123b7a46fa19fa75ef214e8edb92e15ef635e5ba
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>
(cherry picked from commit 7620c7b26ec19e902592baf7f7c4185392c2b15d)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/Qt6WasmMacros.cmake

@@ -23,6 +23,14 @@ function(_qt_internal_wasm_add_target_helpers target)
 
         set(APPNAME ${_target_output_name})
 
+        # Shared library builds preload plugins and qml imports by default.
+        # The json files are generated by scripts in qtbase/util/wasm/preload
+        if (QT_FEATURE_shared)
+            set(PRELOAD "preload: ['qt_plugins.json', 'qt_qml_imports.json'],")
+        else()
+            set(PRELOAD "")
+        endif()
+
         get_target_property(target_output_directory ${target} RUNTIME_OUTPUT_DIRECTORY)
 
         get_target_property(is_test ${target} _qt_is_test_executable)

src/plugins/platforms/wasm/wasm_shell.html

@@ -59,6 +59,7 @@
                         },
                         entryFunction: window.createQtAppInstance,
                         containerElements: [screen],
+                        @PRELOAD@
                     }
                 });
             } catch (e) {