Moving markdown to corelibs (#16916)

* Moving markdown to corelibs

* Fix golangci-lint problem

* Renaming corelibs to pkg

* Fixing golangci-lint

* Renaming from pkg to shared

* Fixing gofmt

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

shared/markdown/document.go

@@ -0,0 +1,22 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package markdown
+
+type Document struct {
+	blockBase
+
+	Children []Block
+}
+
+func (b *Document) Continuation(indentation int, r Range) *continuation {
+	return &continuation{
+		Indentation: indentation,
+		Remaining:   r,
+	}
+}
+
+func (b *Document) AddChild(openBlocks []Block) []Block {
+	b.Children = append(b.Children, openBlocks[0])
+	return openBlocks
+}