Implement Brigadier Mojang API

This is the start of a new module for Paper to add support for API's
that interface Mojang API's directly.

This allows us to version properly by MC version incase Mojang makes any major breaking changes.

It also lets us separate Mojang API's from Paper-API so our downstream friends at Glowstone
will not have to worry about Mojang code.

Adds AsyncPlayerSendCommandsEvent
  - Allows modifying on a per command basis what command data they see.

Adds CommandRegisteredEvent
  - Allows manipulating the CommandNode to add more children/metadata for the client

Spigot-Server-Patches/POM-Changes.patch

@@ -5,7 +5,7 @@ Subject: [PATCH] POM Changes
 
 
 diff --git a/pom.xml b/pom.xml
-index def28b3bec..3dc6c2a3f5 100644
+index def28b3bec..23f33d0477 100644
 --- a/pom.xml
 +++ b/pom.xml
 @@ -0,0 +0,0 @@
@@ -46,6 +46,12 @@ index def28b3bec..3dc6c2a3f5 100644
 -            <artifactId>spigot-api</artifactId>
 +            <groupId>com.destroystokyo.paper</groupId>
 +            <artifactId>paper-api</artifactId>
++            <version>${project.version}</version>
++            <scope>compile</scope>
++        </dependency>
++        <dependency>
++            <groupId>com.destroystokyo.paper</groupId>
++            <artifactId>paper-mojangapi</artifactId>
              <version>${project.version}</version>
              <scope>compile</scope>
          </dependency>