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

examples/vweb/file_upload/index.html

@@ -0,0 +1,6 @@
+<h2>File Upload</h2>
+
+<form method="POST" enctype="multipart/form-data" action="/upload">
+	<input type="file" name="upfile" multiple>
+    <input type="submit" value="Press">
+</form>

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>

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()
+}