github: Create CODEOWNERS file (#457)

.github/CODEOWNERS

diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS
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index 0000000..37502e3
--- /dev/null
+++ b/.github/CODEOWNERS
@@ -0,0 +1,3 @@
+# This codebase has shared ownership and responsibility.
+
+* @hashicorp/terraform-core @hashicorp/terraform-devex @hashicorp/tf-editor-experience-engineers