patch 8.1.1930: cannot recognize .jsx and .tsx files

Problem:    Cannot recognize .jsx and .tsx files.
Solution:   Recognize them as javascriptreact and typescriptreact.
            (closes #4830)
diff --git a/runtime/ftplugin/javascriptreact.vim b/runtime/ftplugin/javascriptreact.vim
new file mode 100644
index 0000000..ea5c9f5
--- /dev/null
+++ b/runtime/ftplugin/javascriptreact.vim
@@ -0,0 +1,2 @@
+" Placeholder for backwards compatilibity: .jsx used to stand for JavaScript.
+source %:h/javascript.vim