Add hcl extension

Signed-off-by: CrazyMax <google@crazymax.dev>

testdata/initial/file.hcl

@@ -0,0 +1,8 @@
+group "default" {
+  targets = ["build"]
+}
+
+target "build" {
+  dockerfile = "./Dockerfile"
+  output = ["type=docker"]
+}