v.gen.js, os_js: port the OS module to JS (#10872)

vlib/os_js/os.v

@@ -0,0 +1,13 @@
+module os_js
+
+pub const (
+	// todo(playX): NodeJS does not seem to have any path limit?
+	max_path_len = 4096
+)
+
+pub struct Result {
+pub:
+	exit_code int
+	output    string
+	// stderr string // TODO
+}