fix: automatically update tflint.hcl (#90)

# Description

The standard `tflint-plugin` updates `.tflint.hcl` only, but we use a
different naming here (without the leading dot).

This PR adds our file pattern to Renovate to enable automatic updates on
this file as well.

# Verification

None.

default.json

@@ -67,5 +67,9 @@
       ],
       "versioningTemplate": "{{#if versioning}}{{{versioning}}}{{else}}semver{{/if}}"
     }
-  ]
+  ],
+
+  "tflint-plugin": {
+    "fileMatch": ["tflint\\.hcl$"]
+  }
 }