Use relative paths in the recommended .gitignore

The reason I'm sending this PR is that ignition creates an `artifacts` folder that you may want to commit, and our `.gitignore` was making that tricky.

Author: alcuadrado

packages/hardhat-core/recommended-gitignore.txt

@@ -1,11 +1,14 @@
 node_modules
 .env
-coverage
-coverage.json
-typechain
-typechain-types
 
 # Hardhat files
-cache
-artifacts
+/cache
+/artifacts
 
+# TypeChain files
+/typechain
+/typechain-types
+
+# solidity-coverage files
+/coverage
+/coverage.json