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

examples/vweb/file_upload/vweb_example.v

@@ -0,0 +1,33 @@
+module main
+
+import vweb
+
+const (
+	port = 8082
+)
+
+struct App {
+	vweb.Context
+}
+
+fn main() {
+	vweb.run<App>(port)
+}
+
+pub fn (mut app App) index() vweb.Result {
+	return $vweb.html()
+}
+
+[post]
+['/upload']
+pub fn (mut app App) upload() vweb.Result {
+	fdata := app.files['upfile']
+
+	mut files := []vweb.RawHtml{}
+
+	for d in fdata {
+		files << d.data.replace_each(['\n', '<br>', '\n\r', '<br>' '\t', '	', ' ', '&nbsp;'])
+	}
+
+	return $vweb.html()
+}