Add events to support scene group substitution

[longfruit]

Description

Add new events to support scene group substitution:

Event	Called when
SceneBlockLoadedEvent	SceneBlock finishes reading from block script
SceneMetaLoadEvent	SceneScriptManager starts loading SceneMeta

Example plugin using these events: SceneHook. Disclaimer: I wrote it.

Basic flow:

1. Plugin provides replacement groups.
2. Replacement groups can be registered on demand or automatically at plugin load time.
   1. On SceneBlock load, plugin can change existing and add new groups.
   2. On SceneMeta load, plugin can hook in dynamic group loads when scene finishes loading.
3. Groups become replaced on demand or automatically at scene load time.

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Also, fix a script issue caused by EVENT_ANY_MONSTER_DIE firing at the same time.

Before	After

Issues fixed by this PR

Type of changes

• Bug fix
• New feature
• Enhancement
• Documentation

Checklist:

• My code follows the style guidelines of this project
• My pull request is unique and no other pull requests have been opened for these changes
• I have read the Contributing note and Code of conduct
• I am responsible for any copyright issues with my code if it occurs in the future.