feat!: remove 3.x compatability layer (#1063)

This PR will remove the browser compatability layer

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Removes the deprecated 3.x browser compatibility layer and its
build/test surface.
> 
> - Deletes `src/compat/*` (compat client, options, emitter, helpers)
and `__tests__/compat/LDClientCompatImpl.test.ts`
> - Removes compat entrypoints from `package.json` `exports` and from
`tsup.config.ts` (no more `compat` bundle)
> - Build now only targets `src/index.ts`; package no longer exposes
`./compat`
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
3964c52. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: joker23

packages/sdk/browser/__tests__/compat/LDClientCompatImpl.test.ts

@@ -30,16 +30,6 @@
         "types": "./dist/index.d.ts",
         "import": "./dist/index.js"
       }
-    },
-    "./compat": {
-      "require": {
-        "types": "./dist/compat.d.cts",
-        "require": "./dist/compat.cjs"
-      },
-      "import": {
-        "types": "./dist/compat.d.ts",
-        "import": "./dist/compat.js"
-      }
     }
   },
   "files": [