mediawiki.action.edit.preview: Use .textSelection( 'getContents' )
Replace .val() by .textSelection( 'getContents' )
for '#wpTextbox1' and '#wpSummary'.
This is already supported by extension CodeEditor.
$( mw ).trigger( 'LivePreviewPrepare' ) is not needed anymore.
Bug: T78838
Change-Id: I30bd709d8b4778f45f3d5f395201e0ca6ce18891