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/shared/timezones/timezones.go

@@ -0,0 +1,29 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package timezones
+
+type Timezones struct {
+	supportedZones []string
+}
+
+func New() *Timezones {
+	timezones := Timezones{}
+
+	timezones.supportedZones = DefaultSupportedTimezones
+
+	return &timezones
+}
+
+func (t *Timezones) GetSupported() []string {
+	return t.supportedZones
+}
+
+func DefaultUserTimezone() map[string]string {
+	defaultTimezone := make(map[string]string)
+	defaultTimezone["useAutomaticTimezone"] = "true"
+	defaultTimezone["automaticTimezone"] = ""
+	defaultTimezone["manualTimezone"] = ""
+
+	return defaultTimezone
+}