Add API Version flag for 1.13+ Bukkit plugins (#462)

Fixes GH-453

Author: Zach

src/main/kotlin/com/demonwav/mcdev/platform/bukkit/BukkitTemplate.kt

@@ -101,6 +101,14 @@ object BukkitTemplate {
             properties.setProperty("HAS_WEBSITE", "true")
         }
 
+        // Plugins targeting 1.13 or newer need an explicit api declaration flag
+        // Unfortunately this flag has no contract to match any specific API version
+        val mcVer = settings.minecraftVersion.substring(0, 4).toDoubleOrNull()
+        if (mcVer != null && mcVer >= 1.13) {
+            properties.setProperty("API_VERSION", "1.13")
+            properties.setProperty("HAS_API_VERSION", "true")
+        }
+
         BaseTemplate.applyTemplate(project, file, MinecraftFileTemplateGroupFactory.BUKKIT_PLUGIN_YML_TEMPLATE, properties, true)
     }
 }

src/main/resources/fileTemplates/j2ee/bukkit_plugin_description_file.yml.ft

@@ -1,6 +1,9 @@
 name: ${NAME}
 version: ${VERSION}
 main: ${MAIN}
+#if (${HAS_API_VERSION})
+api-version: ${API_VERSION}
+#end
 #if (${HAS_PREFIX})
 prefix: ${PREFIX}
 #end