Refactor C#-defined command system

For now, when defining a command in C# code, I need to add it to two places:
1. `TexFormulaParser.commands` hash set.
2. `TexFormulaParser.ProcessCommand` method.

It looks like a lot of duplication comes from this, and also this system is not user-extendable.

What I imagine is the following:

1. We need to declare one command dictionary, no name duplication should ever be used.
2. Create a `TexCommand` class and allow the implementation to deal with parsed arguments etc.
3. Add an external `RegisterCommand` API for our users to define their own commands in C#.