Make text effects and modifiers modular

[Jowan-Spooner]

Removes the text effect and modifier logic from the DialogText node.

The index.gd file can now register text effects and modifiers.
These are effectively just methods. The text subsystem collects them and applies/reads them to/from the text before sending it to the DialogText node. Then the DialogText node calls execute_effects on every letter advance.

Text modifiers are also methods, these are only called once before the text is shown.

The text effect methods have access to the calling dialog_text node, the skipping boolean and the argument string.

All effects/modifiers have been moved to the text subsystem except portrait which is now part of the portrait subsystem.

Additionally the syntax for random selection has been changed from [option1, option2] to [option1/option2].

This PR improves little to nothing for the average user, but makes it easier

• to maintain existing text effects/modifiers in the future
• to add text effects/modifiers both in core and in additions
• to create a custom dialog node implementation (because less functionality is in the default one)