Creating a dedicated package for iframe API typings

If users are willing to use Typescript to develop scripts for maps, they will need typings for the `WA` object.
This commit creates a new package (@workadventure/iframe-api-typings) that contains only the `iframe_api.d.ts` file.

The file is generated from the build of the front and isolated in this package.
This is necessary because the iframe_api.js file is supposed to always be loaded from WorkAdventure directly (and there is no @workadventure/iframe-api package and there will never be one)

.github/workflows/push-to-npm.yml

@@ -0,0 +1,55 @@
+name: Push @workadventure/iframe-api-typings to NPM
+on:
+  # TODO remove action on push
+  push: ~
+  release:
+    types: [created]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      # Setup .npmrc file to publish to npm
+      - uses: actions/setup-node@v2
+        with:
+          node-version: '14.x'
+          registry-url: 'https://registry.npmjs.org'
+      # TODO: replace version with RELEASE tag
+      # TODO: enable declaration flag in tsconfig dynamically
+      - name: Install Protoc
+        uses: arduino/setup-protoc@v1
+        with:
+          version: '3.x'
+
+      - name: "Install dependencies"
+        run: yarn install
+        working-directory: "front"
+
+      - name: "Install messages dependencies"
+        run: yarn install
+        working-directory: "messages"
+
+      - name: "Build proto messages"
+        run: yarn run proto && yarn run copy-to-front
+        working-directory: "messages"
+
+      - name: "Create index.html"
+        run: ./templater.sh
+        working-directory: "front"
+
+      - name: "Build"
+        run: yarn run build
+        env:
+          API_URL: "localhost:8080"
+        working-directory: "front"
+      # We build the front to generate the typings of iframe_api, then we copy those typings in a separate package.
+      - name: Copy typings to package dir
+        run: cp front/dist/src/iframe_api.d.ts front/packages/iframe-api-typings/iframe_api.d.ts
+      - name: Install dependencies in package
+        run: yarn install
+        working-directory: "front/packages/iframe-api-typings"
+      - name: Publish package
+        run: yarn publish
+        working-directory: "front/packages/iframe-api-typings"
+        env:
+          NODE_AUTH_TOKEN: ${{ secrets.NPM_TOKEN }}