Add devcontainer.json

* adds `devcontainer.json` for codespace users
* We install foundry packages using use `"postCreateCommand"` commands
* See TODO and FIXME comments for later updates to `devcontainer.json`

test/Gas.UnitTests.t.sol

@@ -196,5 +196,5 @@ contract GasTest is Test {
         vm.stopPrank();
     }
 
-    function testGetPaymentStatus(address sender) public {
+    function testGetPaymentStatus(address sender) public {}
 }