feat: warn about unknown experimental input

When the user specifies unknown properties in the experimental input,
then we should inform them about it. It does not have to stop the action
from working, but a logged warning makes sense.

src/main.ts

@@ -3,6 +3,7 @@ import { Backport, Config, experimentalDefaults } from "./backport";
 import { Github } from "./github";
 import { Git } from "./git";
 import { execa } from "execa";
+import dedent from "dedent";
 
 /**
  * Called from the action.yml.
@@ -30,6 +31,14 @@ async function run(): Promise<void> {
     return;
   }
 
+  for (const key in experimental) {
+    if (!(key in experimentalDefaults)) {
+      console.warn(dedent`Encountered unexpected key in input 'experimental'.\
+        No experimental config options known for key '${key}'.\
+        Please check the documentation for details about experimental features.`);
+    }
+  }
+
   const github = new Github(token);
   const git = new Git(execa);
   const config: Config = {