Use TypeScript source for development, swap to build during release

This changes the JS SDK to point `main` to TypeScript source and remove any
indication of `typings`. For local development and CI workflows, it means many
steps can run without building first, which saves lots of time.

During release, we still build for Node and browsers as before. The release
script adjusts the `main` and `typings` fields before publishing and
distribution to point to the built output for those that use them.

package.json

@@ -27,9 +27,10 @@
     "matrix-gen-i18n": "scripts/gen-i18n.js",
     "matrix-prune-i18n": "scripts/prune-i18n.js"
   },
-  "main": "./lib/index.js",
-  "typings": "./lib/index.d.ts",
+  "main": "./src/index.js",
   "matrix_src_main": "./src/index.js",
+  "matrix_lib_main": "./lib/index.js",
+  "matrix_lib_typings": "./lib/index.d.ts",
   "scripts": {
     "prepublishOnly": "yarn build",
     "i18n": "matrix-gen-i18n",