Secspressions!

[Moderocky]

Description

Adds a special type of expression, the SectionExpression!

SectionExpressions are like simple expressions, but they are allowed to start a section when used in a compatible effect.

This means that a regular effect can be followed by a section iff it contains a section expression.
The expression is allowed to determine the parsing/loading of the following section.

Its only difference from a regular expression is that it may be given a (nullable) SectionNode and List<TriggerItem> during init as an EffectSection would.

Note: this is just API, no new syntax is included in this PR.

More Info

Q: Does this change any existing syntax?
A: No.

Q: Does a section expression need to have a section?
A: No, like an EffectSection it could be used as a regular expression, but this is up to the implementation.

Q: What are the possible uses of this?
A: It's mainly offered for addons, but potentially future syntax. You can easily create 'lambda'-style things like below:

set {_my sword} to a diamond sword:
    set the item's name to "swordzie"
give {_my sword} to player

set {_runnable} to a new runnable:
    broadcast "hello"
# The expression can choose to run the code when it likes, or never

run {_runnable} # broadcasts hello
run {_runnable} # broadcasts hello

set {_java} to some java code:
    System.out.println("hello");
# The section actually has authority over how/whether to parse the raw text node
# so you can make pretty much anything with it

Note: these are just examples of what people might do with the API. None are included in this pull request.

Q: What happens if I use a section expression inside a line that's already a section header, like a condition/loop/while? E.g. loop <my section expression>:
A: This will produce a parse-time error.

Q: What happens if I use multiple section expressions in one line? E.g. teleport <my section expression> to <my section expression>:
A: This will produce a parse-time error.

Q: What happens if I add a colon : and indent code, but have no section expression/condition/regular section before it?
A: This will produce a parse-time error.

Q: Is there a restriction on where I can use section expressions?
A: Technically no, as long as: A) it's not in a regular section line, B) there isn't already a section expression in the line. Obviously they look better in some lines (set) than others (teleport or whatever) but there is deliberately no restriction on this so that addons can implement them how they like.

Q: Is code in the section automatically run?
A: No. Everything is left up to the expression, as in an EffectSection. That means you can run the code when the expression is requested, run it later, never run it, or even have something that isn't code in the section. You get the raw SectionNode during init, and complete freedom how to process it.

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Target Minecraft Versions: any
Requirements: none
Related Issues: #6536, closes #4472

[sovdeeth]

Looks good but i'd like to see more documentation on the new classes

[UnderscoreTud]

Is this used anywhere?

[APickledWalrus]

Looking good

[APickledWalrus]

A few final touches

[APickledWalrus]

Should be good to go now