Fix markdown preview $$ support (#31514)

close #31481

currently `$$A + B$$ test` will ignore text after $$ block

test text

![圖片](39b2974b-c0b6-48a0-87d0-5f4a13615eed)

before fix

![圖片](15469e4c-474d-4128-b46f-d6cadaafbd68)

after fix

![圖片](c1025eef-177f-4ade-988f-510e7039f3f9)

github display

![圖片](97cd1e10-ac94-4899-86d8-8e359ef1d694)

(cherry picked from commit f0033051d5ce07f5a18d2aacef6bfdca5fc69014)

modules/markup/markdown/math/inline_block_node.go

@@ -0,0 +1,31 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package math
+
+import (
+	"github.com/yuin/goldmark/ast"
+)
+
+// InlineBlock represents inline math e.g. $$...$$
+type InlineBlock struct {
+	Inline
+}
+
+// InlineBlock implements InlineBlock.
+func (n *InlineBlock) InlineBlock() {}
+
+// KindInlineBlock is the kind for math inline block
+var KindInlineBlock = ast.NewNodeKind("MathInlineBlock")
+
+// Kind returns KindInlineBlock
+func (n *InlineBlock) Kind() ast.NodeKind {
+	return KindInlineBlock
+}
+
+// NewInlineBlock creates a new ast math inline block node
+func NewInlineBlock() *InlineBlock {
+	return &InlineBlock{
+		Inline{},
+	}
+}