vweb: add multipart/form-data parser and file upload (#8160)

examples/vweb/file_upload/upload.html

@@ -0,0 +1,14 @@
+<meta charset="utf-8">
+
+File amount: @fdata.len
+
+@for i, data in fdata
+
+<h2>Filename: @data.filename</h2>
+<h2>Type: @data.content_type</h2>
+
+<p>@{files[i]}</p>
+
+@end
+
+<a href="/">Back</a>