[ci skip] add gitpod config

.gitpod.Dockerfile

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+FROM gitpod/workspace-full
+
+USER gitpod
+
+RUN brew install kotlin

.gitpod.yml

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+image:
+  file: .gitpod.Dockerfile
+tasks:
+  - init: gradle compileKotlin
+vscode:
+  extensions:
+    - mathiasfrohlich.Kotlin@1.7.0:9xQZtwTUg4bdXHCMyxM7vQ==
+    - fwcd.kotlin@0.2.11:moh8IDanzsIlhtK2IeiLmQ==