Port of godotengine/godot#44594 to Godot 3.x.

Co-authored-by: Shatur95 <genaloner@gmail.com>
Co-authored-by: lupoDharkael <izhe@hotmail.es>

Not properly supported in Godot 3.x.

Note: this could be ported to simple `PackedStringArray` equality comparison in Godot 4.0.

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Consistent with other parsers in Godot, such as json/xml.

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Consistent with `Expression.get_error_text()`, and `CommandLineParser`'s own `get_help_text()`.

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`get_values/prefixes()` to `get_value/prefix_list()`, make consistent with other `_list()` methods on Godot.

`get_forwarded_args()` to `get_forwarding_args()`, make consistent with `allow_forwarding_args` property.

These are not so trivial to implement in Godot 3.x since there's no Callable, which is Godot 4.x feature, and it might be better to see whether checher functionality is really needed in the first place, since validation is also possible to do via code after parsing, which is user-specific.

…xt()`

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There's not much benefit in separating the two, and in most cases makes API more confusing. Current parsing code validates arguments as well anyways.

Allows to conveniently add new options without instantiating `CommandLineOption` classes manually.

…tion()`

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When declarative approach is not enough.