feat: add Static, StaticFS and SPA file serving

- Add Static(prefix, root) to serve files from disk on Kite and Group
- Add StaticFS(prefix, fs) to serve from any http.FileSystem on Kite and Group
- Add SPA(root) and SPAFS(fs) for Single Page Application serving — falls back to index.html for unknown paths so client-side routing works correctly

group.go

@@ -8,6 +8,21 @@ type Group struct {
 	mws    []Middleware
 }
 
+func (g *Group) Static(prefix, root string) {
+	g.StaticFS(prefix, http.Dir(root))
+}
+
+func (g *Group) StaticFS(prefix string, fs http.FileSystem) {
+	if prefix == "" || prefix[len(prefix)-1] != '/' {
+		prefix += "/"
+	}
+	fullPrefix := g.prefix + prefix
+
+	handler := http.StripPrefix(fullPrefix, http.FileServer(fs))
+	g.k.r.Handler(http.MethodGet, fullPrefix+"*filepath", handler)
+	g.k.r.Handler(http.MethodHead, fullPrefix+"*filepath", handler)
+}
+
 func (g *Group) Group(prefix string, mws ...Middleware) *Group {
 	return &Group{
 		k:      g.k,