Add gitpod config (#8921)

.gitpod.Dockerfile

@@ -0,0 +1,8 @@
+# See https://www.gitpod.io/docs/java-in-gitpod/ for a full documentation of Java in GitPod
+
+FROM gitpod/workspace-full
+
+# All available versions can be listed using sdk ls java
+# More information about SDKMAN available at https://github.com/sdkman/sdkman-cli#sdkman-cli
+RUN bash -c ". /home/gitpod/.sdkman/bin/sdkman-init.sh \
+             && sdk install java 18.0.1.1-open)"

.gitpod.yml

@@ -0,0 +1,8 @@
+image:
+  file: .gitpod.Dockerfile
+tasks:
+  - init: ./gradlew assemble
+vscode:
+  extensions:
+    - redhat.java
+    - vscjava

README.md

@@ -71,7 +71,7 @@ An explanation of donation possibilities and usage of donations is available at
 [![OpenHub](https://www.openhub.net/p/jabref/widgets/project_thin_badge.gif)](https://www.openhub.net/p/jabref)
 [![Deployment Status](https://github.com/JabRef/jabref/workflows/Deployment/badge.svg)](https://github.com/JabRef/jabref/actions?query=workflow%3ADeployment)
 [![Test Status](https://github.com/JabRef/jabref/workflows/Tests/badge.svg)](https://github.com/JabRef/jabref/actions?query=workflow%3ATests)
-[![codecov.io](https://codecov.io/github/JabRef/jabref/coverage.svg?branch=master)](https://codecov.io/github/JabRef/jabref?branch=master)
+[![codecov.io](https://codecov.io/github/JabRef/jabref/coverage.svg?branch=master)](https://codecov.io/github/JabRef/jabref?branch=main)
 
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