Refactor markup package (#32399)

To make the markup package easier to maintain:
1. Split some go files into small files
2. Use a shared util.NopCloser, remove duplicate code
3. Remove unused functions

modules/markup/render_helper.go

@@ -0,0 +1,21 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package markup
+
+import (
+	"context"
+	"html/template"
+)
+
+// ProcessorHelper is a helper for the rendering processors (it could be renamed to RenderHelper in the future).
+// The main purpose of this helper is to decouple some functions which are not directly available in this package.
+type ProcessorHelper struct {
+	IsUsernameMentionable func(ctx context.Context, username string) bool
+
+	ElementDir string // the direction of the elements, eg: "ltr", "rtl", "auto", default to no direction attribute
+
+	RenderRepoFileCodePreview func(ctx context.Context, options RenderCodePreviewOptions) (template.HTML, error)
+}
+
+var DefaultProcessorHelper ProcessorHelper