fix(Scripts/Naxxramas): refactored instance script

[Razor2142]

Changes Proposed:

This PR proposes changes to:

• Core (units, players, creatures, game systems).
• Scripts (bosses, spell scripts, creature scripts).
• Database (SAI, creatures, etc).

Issues Addressed:

• Fixed Grobbulus gate state when in progress.
• Fixed Kel'Thuzad RP dialog when Mr. Bigglesworth is killed.

SOURCE:

The changes have been validated through:

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Tests Performed:

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• Tested in-game by the author.
• Tested in-game by other community members/someone else other than the author/has been live on production servers.
• This pull request requires further testing and may have edge cases to be tested.

How to Test the Changes:

• This pull request can be tested by following the reproduction steps provided in the linked issue
• This pull request requires further testing. Provide steps to test your changes. If it requires any specific setup e.g multiple players please specify it as well.

Known Issues and TODO List:

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[Nyeriah]

Looks good, I'm just not sure about moving the loading of the script files away from the loader file

[Razor2142]

@Nyeriah Feels cleaner to have the instance self contained but I can revert it.

[Nyeriah]

Feels cleaner to have the instance self contained but I can revert it.

It would make the review process faster without it, as I am not certain that is something we want at this time as it wouldn't be consistent with the rest of the code. So if you could keep that as it was, I think I am ready to green light this. We could, of course, always come back to that in another PR, if we decide we are up to changing all other instances too