Place .d.cts files in types/

This is both tidier and ensures they're included in the package. Also rename the import in main.d.cts.

Author: novemberborn

index.d.cts

@@ -0,0 +1,3 @@
+// FIXME: Self-refencing "ava" should work, but seems to fail.
+import test from "../index.js";
+export = test

plugin.d.cts

@@ -0,0 +1,2 @@
+// FIXME: Self-refencing "ava/plugin" should work, but seems to fail.
+export * from "../plugin.js";