6.x - Move help option to help register file

[flakey5]

What does this PR do?

Resolves a todo specified in core/modules/main/core_options.lua line 15. Relocates the --help command line argument to a _register.lua located in the help module and has the core_options.lua file register it.

How does this PR change Premake's behavior?

No noticeable or breaking change. Internally the help module would be registered rather than defining the command line argument with the rest of the main arguments.

Anything else we should know?

I wasn't too sure where to actually register the help module so I just did it in the same file it was defined in originally.

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
• Minimize the number of commits
• Align documentation to your changes

You can now support Premake on our OpenCollective. Your contributions help us spend more time responding to requests like these!