Expose public/ API as submodule (#23345)

* model -> public/model

* plugin -> public/plugin

* public/model/utils -> public/utils

* platform/shared/mlog -> public/shared/mlog

* platform/shared/i18n -> public/shared/i18n

* platform/shared/markdown -> public/shared/markdown

* platform/services/timezones -> public/shared/timezones

* channels/einterfaces -> einterfaces

* expose public/ submodule

* go mod tidy

* .github: cache-dependency-path, setup-go-work

* modules-tidy for public/ too

* remove old gomodtidy

server/public/model/channel_mentions.go

@@ -0,0 +1,28 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+import (
+	"regexp"
+	"strings"
+)
+
+var channelMentionRegexp = regexp.MustCompile(`\B~[a-zA-Z0-9\-_]+`)
+
+func ChannelMentions(message string) []string {
+	var names []string
+
+	if strings.Contains(message, "~") {
+		alreadyMentioned := make(map[string]bool)
+		for _, match := range channelMentionRegexp.FindAllString(message, -1) {
+			name := match[1:]
+			if !alreadyMentioned[name] {
+				names = append(names, name)
+				alreadyMentioned[name] = true
+			}
+		}
+	}
+
+	return names
+}