Allow set flags to be manipulate via keywords

This adds support for updating an existing region, so that
if you have a long list of items in a set flag, such as
`deny-spawn` you can add or remove items, rather than overwrite.

Example:
```
/rg flag test deny-spawn add creeper
/rg flag test deny-spawn remove zombie
```

worldguard-bukkit/build.gradle.kts

@@ -11,7 +11,6 @@ applyPlatformAndCoreConfiguration()
 applyShadowConfiguration()
 
 repositories {
-    mavenLocal()
     maven {
         name = "paper"
         url = uri("https://papermc.io/repo/repository/maven-public/")
@@ -28,7 +27,7 @@ dependencies {
     "api"("com.destroystokyo.paper:paper-api:1.15-R0.1-SNAPSHOT")
     "implementation"("io.papermc:paperlib:1.0.2")
     "api"("com.sk89q.worldedit:worldedit-bukkit:${Versions.WORLDEDIT}") { isTransitive = false }
-    "implementation"("com.sk89q:commandbook:3.0-SNAPSHOT") { isTransitive = false }
+    "implementation"("com.sk89q:commandbook:2.3") { isTransitive = false }
     "implementation"("org.bstats:bstats-bukkit:1.5")
 }
 

worldguard-bukkit/src/main/java/com/sk89q/worldguard/bukkit/listener/WorldGuardCommandBookListener.java

@@ -19,7 +19,7 @@
 
 package com.sk89q.worldguard.bukkit.listener;
 
-import com.sk89q.commandbook.component.info.InfoComponent;
+import com.sk89q.commandbook.InfoComponent;
 import com.sk89q.worldguard.LocalPlayer;
 import com.sk89q.worldguard.WorldGuard;
 import com.sk89q.worldguard.bukkit.WorldGuardPlugin;

worldguard-core/src/main/java/com/sk89q/worldguard/protection/flags/SetFlag.java

@@ -20,18 +20,25 @@
 package com.sk89q.worldguard.protection.flags;
 
 import com.google.common.collect.Sets;
+import com.sk89q.worldguard.protection.regions.ProtectedRegion;
 
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Objects;
 import java.util.Set;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
 
 /**
  * Stores a set of types.
  */
 public class SetFlag<T> extends Flag<Set<T>> {
 
+    private static final Pattern ANY_MODIFIER = Pattern.compile("^(add|sub|subtract|rem|remove) (.*)$");
+    private static final Pattern REMOVE_MODIFIERS = Pattern.compile("^(sub|subtract|rem|remove) (.*)$");
+
     private Flag<T> subFlag;
 
     public SetFlag(String name, RegionGroup defaultGroup, Flag<T> subFlag) {
@@ -60,10 +67,32 @@ public class SetFlag<T> extends Flag<Set<T>> {
             return Sets.newHashSet();
         } else {
             Set<T> items = Sets.newHashSet();
+            boolean subtractive = false;
+
+            // If the input starts with particular keywords, attempt to load the existing values,
+            // and make this a modification, instead of an overwrite.
+            Matcher keywordMatcher = ANY_MODIFIER.matcher(input);
+            if (keywordMatcher.matches()) {
+                ProtectedRegion region = Objects.requireNonNull((ProtectedRegion) context.get("region"));
+
+                Set<T> existingValue = region.getFlag(this);
+                if (existingValue != null) {
+                    items.addAll(existingValue);
+                }
+
+                subtractive = REMOVE_MODIFIERS.matcher(input).matches();
+                input = keywordMatcher.group(2);
+            }
 
             for (String str : input.split(",")) {
                 FlagContext copy = context.copyWith(null, str, null);
-                items.add(subFlag.parseInput(copy));
+
+                T subFlagValue = subFlag.parseInput(copy);
+                if (subtractive) {
+                    items.remove(subFlagValue);
+                } else {
+                    items.add(subFlagValue);
+                }
             }
 
             return items;