Add Gradle wrapper and CI workflow

- Commit gradlew, gradlew.bat and gradle/wrapper so ./gradlew build works
  from a fresh clone as the README instructs (Gradle 8.7)
- Fix .gitignore rule order: *.jar came after the wrapper-jar negation and
  the last matching rule wins, so the wrapper jar was still ignored
- CI on push and pull request: build (grammar with -Werror), test, and
  parseExamples

gradle/wrapper/gradle-wrapper.properties

@@ -0,0 +1,7 @@
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.7-bin.zip
+networkTimeout=10000
+validateDistributionUrl=true
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists