Add empty root build.gradle so build-system detectors see Gradle

The Burp BApp Store submission bot file-checks the repo root for
build.gradle / pom.xml and rejects multi-project layouts where the
real build script lives in a subdirectory. The actual build config
stays in web3-decoder/build.gradle; this file just makes the root
recognisable as a Gradle project. ./gradlew jar at the root already
delegates to :web3-decoder:jar via the existing multi-project setup.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: gry

build.gradle

@@ -0,0 +1,6 @@
+// Multi-project Gradle build. The buildable Burp extension lives in the
+// :web3-decoder subproject (see settings.gradle and web3-decoder/build.gradle).
+//
+// From the repo root, `./gradlew jar` builds web3-decoder/build/libs/web3-decoder.jar.
+// This root build script is intentionally empty — it exists so build-system
+// detectors (e.g. the BApp Store submission bot) recognise the repo as Gradle.