companion: port to TypeScript (#6179)

Supersedes #6178.

This branch keeps the exact same final content as #6178, but splits
history for reviewability:

1. `chore(companion): rename source and test files from .js to .ts`
(pure `git mv`, no content changes)
2. `feat(companion): port Companion to TypeScript` (actual
content/type/schema/build updates)

Validation note:
- Final tree is byte-for-byte identical to `ts-companion`
(`7b5b16a298690b0492de4cc4fab744f998b45570`).

---------

Co-authored-by: Mikael Finstad <finstaden@gmail.com>

biome.json

@@ -67,6 +67,18 @@
       }
     }
   },
+  "overrides": [
+    {
+      "includes": ["packages/@uppy/companion/**"],
+      "linter": {
+        "rules": {
+          "complexity": {
+            "useLiteralKeys": "off"
+          }
+        }
+      }
+    }
+  ],
   "javascript": {
     "formatter": {
       "quoteStyle": "single",