Major improvements to command manager

• Added "assert" tag to command file parser, which acts similarly to "context" tag but doesn't pass the argument to the method hook. Useful for when you just need to assert that certain context is true or exists, but don't need the actual data beyond that.
• Created ArgType, which is the exact same as CommandArgumentType. CommandArgumentType can make your lines of code quite long, so ArgType just acts as an alias.
• Made CommandCollection coalesce multiple base commands with the same names/aliases into a single command which will attempt to run all of the commands until one is successful. This behavior is now consistent with how subcommands always worked, and simplifies command writing further.