add gitpod config (#20995)

Add a config for <https://www.gitpod.io/>. This allows users to easily
run an online IDE pre-configured to develop and test Gitea by opening
<https://gitpod.io/#https://github.com/go-gitea/gitea>.

[![Open in
Gitpod](https://gitpod.io/button/open-in-gitpod.svg)](https://gitpod.io/#https://github.com/go-gitea/gitea)

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

.gitpod.yml

@@ -0,0 +1,34 @@
+image: gitpod/workspace-full:latest
+
+tasks:
+  - name: Setup
+    init: |
+      make deps
+      make build
+    command: |
+      gp sync-done setup
+      exit 0
+  - name: Run frontend
+    command: |
+      gp sync-await setup
+      make watch-frontend
+  - name: Run backend
+    command: |
+      gp sync-await setup
+      mkdir -p custom/conf/
+      echo -e "[server]\nROOT_URL=https://3000-$GITPOD_WORKSPACE_ID.${GITPOD_WORKSPACE_CLUSTER_HOST}/" > custom/conf/app.ini
+      echo -e "\n[database]\nDB_TYPE = sqlite3\nPATH = /workspace/gitea/data/gitea.db" >> custom/conf/app.ini
+      export TAGS="sqlite sqlite_unlock_notify"
+      make watch-backend
+
+vscode:
+  extensions:
+    - editorconfig.editorconfig
+    - dbaeumer.vscode-eslint
+    - golang.go
+    - stylelint.vscode-stylelint
+    - DavidAnson.vscode-markdownlint
+
+ports:
+  - name: Gitea
+    port: 3000