chore: add GItHub CI Action for project

.github/scripts/build.sh

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+#!/bin/bash
+
+set -eo pipefail
+
+# build with SwiftPM:
+# https://developer.apple.com/documentation/swift_packages/building_swift_packages_or_apps_that_use_them_in_continuous_integration_workflows
+
+xcodebuild -workspace Mastodon.xcworkspace \
+	-scheme Mastodon \
+	-disableAutomaticPackageResolution \
+	-destination "platform=iOS Simulator,name=iPhone SE (2nd generation)" \
+	clean \
+	build | xcpretty

.github/scripts/setup.sh

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+#!/bin/bash
+
+sudo gem install cocoapods-keys
+pod install

.github/workflows/main.yml

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+name: CI
+
+on:
+  push:
+    branches:
+    - master
+    - develop
+    - feature/*
+  pull_request:
+    branches:
+    - develop
+
+# macOS environments: https://github.com/actions/virtual-environments/tree/main/images/macos
+
+jobs:
+  build:
+    name: CI build
+    runs-on: macos-11.0
+    steps:
+      - name: checkout
+        uses: actions/checkout@v2
+      - name: force Xcode 12.2
+        run: sudo xcode-select -switch /Applications/Xcode_12.2.app
+      - name: setup
+        run: exec ./.github/scripts/setup.sh
+      - name: build
+        run: exec ./.github/scripts/build.sh