ci: Split mainline pipeline and add support for GitHub releases (#2900)

* ci: Add mock build alternative for fast testing

* ci: Always cache build

* ci: Extract steps to download build stage artifacts

* ci: Add template to release to GitHub

* ci: Add template to release to Azure Universal Artifacts

* ci: Split mainline to two pipelines

.ci/yuzu-mainline-step1.yml

@@ -0,0 +1,8 @@
+trigger:
+- master
+
+stages:
+- stage: merge
+  displayName: 'merge'
+  jobs:
+  - template: ./templates/merge.yml