Add .devcontainer (#995)

Author: PopFlamingo

Diff for: .devcontainer/Dockerfile

@@ -0,0 +1 @@
+FROM swiftlang/swift:nightly-5.7-focal

Diff for: .devcontainer/devcontainer.json

@@ -0,0 +1,3 @@
+{
+    "dockerFile": "Dockerfile"
+}