Use a general approach to access custom/static/builtin assets (#24022)

The idea is to use a Layered Asset File-system (modules/assetfs/layered.go)

For example: when there are 2 layers: "custom", "builtin", when access
to asset "my/page.tmpl", the Layered Asset File-system will first try to
use "custom" assets, if not found, then use "builtin" assets.

This approach will hugely simplify a lot of code, make them testable.

Other changes:

* Simplify the AssetsHandlerFunc code
* Simplify the `gitea embedded` sub-command code

---------

Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/public/serve_dynamic.go

@@ -6,17 +6,10 @@
 package public
 
 import (
-	"io"
-	"net/http"
-	"os"
-	"time"
+	"code.gitea.io/gitea/modules/assetfs"
+	"code.gitea.io/gitea/modules/setting"
 )
 
-func fileSystem(dir string) http.FileSystem {
-	return http.Dir(dir)
-}
-
-// serveContent serve http content
-func serveContent(w http.ResponseWriter, req *http.Request, fi os.FileInfo, modtime time.Time, content io.ReadSeeker) {
-	http.ServeContent(w, req, fi.Name(), modtime, content)
+func BuiltinAssets() *assetfs.Layer {
+	return assetfs.Local("builtin(static)", setting.StaticRootPath, "public")
 }