Add gradle build files

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

spring-shell-core/spring-shell-core.gradle

@@ -0,0 +1,18 @@
+plugins {
+	id 'org.springframework.shell.module'
+}
+
+description = 'Spring Shell Core'
+
+dependencies {
+	management platform(project(":spring-shell-management"))
+	implementation project(':spring-shell-table')
+	api('org.springframework:spring-core')
+	api('org.springframework.boot:spring-boot-starter-validation')
+	api('org.springframework:spring-messaging')
+	api('org.jline:jline')
+	api('org.antlr:ST4')
+	testImplementation 'org.springframework.boot:spring-boot-starter-test'
+	testImplementation 'org.awaitility:awaitility'
+	testImplementation 'com.google.jimfs:jimfs'
+}