feat: initialize project and add github actions

初始化项目，添加 GitHub Actions

.github/workflows/gradle.yml

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+name: Java CI
+
+on: 
+  push:
+    branches:
+      - master
+
+jobs:
+  test:
+    name: Algorithms Test
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        java-version: [1.8]
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - name: Checkout project
+        uses: actions/checkout@v2
+        with:
+          ref: master
+      - name: Set up jdk
+        uses: actions/setup-java@v1
+        with:
+          java-version: ${{ matrix.java-version }}
+      - name: JUnit5 tests on ubuntu
+        if: startsWith(matrix.os, 'ubuntu')
+        run: |
+          chmod +x gradlew
+          ./gradlew clean test
+      - name: JUnit5 tests on macos
+        if: startsWith(matrix.os, 'macos')
+        run: |
+          chmod +x gradlew
+          ./gradlew clean test
+      - name: JUnit5 tests on windows
+        if: startsWith(matrix.os, 'windows')
+        run: gradle clean test

.gitignore

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+##----------Android----------
+# build
+*.apk
+*.ap_
+*.dex
+*.class
+bin/
+gen/
+build/
+
+# gradle
+.gradle/
+gradle-app.setting
+!gradle-wrapper.jar
+build/
+
+local.properties
+
+##----------idea----------
+*.iml
+.idea/
+*.ipr
+*.iws
+
+# Android Studio Navigation editor temp files
+.navigation/
+
+##----------Other----------
+# osx
+*~
+.DS_Store
+gradle.properties
+
+.vscode