Set entrypoints to use ./lib rather than ./src (#12906)

Currently, we replace the entrypoints in package.json during the release
cycle. I think, historically, this was done to make element-web development
easier, but that doesn't actually use these entrypoints (instead it imports
from `src`).

Accordingly, I think the switcheroo is unnecessary; furthermore it causes a
whole bunch of confusion by making the development environment different from
the release environment, and it complicates our CI and release process.

In short, the switcheroo has to die.

package.json

@@ -26,10 +26,8 @@
     "engines": {
         "node": ">=20.0.0"
     },
-    "main": "./src/index.ts",
-    "matrix_src_main": "./src/index.ts",
-    "matrix_lib_main": "./lib/index.ts",
-    "matrix_lib_typings": "./lib/index.d.ts",
+    "main": "./lib/index.ts",
+    "typings": "./lib/index.d.ts",
     "matrix_i18n_extra_translation_funcs": [
         "UserFriendlyError"
     ],