Add gradle build files

- First set of changes for introduction of a gradle build.
- Relates #470

spring-shell-core-test-support/spring-shell-core-test-support.gradle

@@ -0,0 +1,11 @@
+plugins {
+	id 'org.springframework.shell.module'
+}
+
+description = 'Spring Shell Core Test Support'
+
+dependencies {
+	management platform(project(":spring-shell-management"))
+	implementation project(':spring-shell-core')
+	api('org.assertj:assertj-core')
+}