Move Flex & Box component into shared component folder (#30357)

* refactor: move Flex component in shared components

* refactor: update imports

* refactor: remove Flex pcss file

* fix: Flex component css override

* test: update snapshots

* fix: html export

* chore: add css module support to jest

* chore: keep old copyright

* refactor: change `mx_Flex` in `ErrorView` to `mx_ErrorView_flexContainer`

* test: update snapshots

* refactor: move Box component in shared components

* refactor: update import and css override

* test: update snapshots

test/unit-tests/components/views/rooms/RoomHeader/__snapshots__/RoomHeader-test.tsx.snap

@@ -3,7 +3,7 @@
 exports[`RoomHeader dm does not show the face pile for DMs 1`] = `
 <DocumentFragment>
   <header
-    class="mx_Flex mx_RoomHeader light-panel"
+    class="flex mx_RoomHeader light-panel"
     style="--mx-flex-display: flex; --mx-flex-direction: row; --mx-flex-align: center; --mx-flex-justify: start; --mx-flex-gap: var(--cpd-space-3x); --mx-flex-wrap: nowrap;"
   >
     <button
@@ -25,7 +25,7 @@ exports[`RoomHeader dm does not show the face pile for DMs 1`] = `
       tabindex="0"
     >
       <div
-        class="mx_Box mx_RoomHeader_info mx_Box--flex"
+        class="mx_RoomHeader_info box-flex"
         style="--mx-box-flex: 1;"
       >
         <div