Use structs instead of NamedTuple's in parse_brackets()

[JamesWrigley]

@KristofferC, @mlechu, this is a more invasive but reliable (I think) solution to #600. Can confirm that it fixes the invalidations from parse_brackets() on 1.12 for InitialValues.jl (though there's a plethora of others). If y'all think it's a good idea I can make a PR to the main repo.

[codecov]

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[c42f]

I think this is fine if we need it. Though I'd prefer a less invasive alternative if we want to keep the code idiomatic for future Julia versions (assuming 1.13 doesn't suffer from this?)

The simplest least invasive way could be to type assert the type of opts as soon as it's returned? Then the performance hacking is localized in one place?

[c42f]

Couple of notes:

1. Why are these options structs mutable?
2. Unfortunately @kwdef doesn't exist on older julia versions

[c42f]

Does this do the trick?

Suggested change

	return ParseUnaryBracketOpts(; needs_parameters=is_paren_call,
	is_paren_call=is_paren_call,
	is_block=!is_paren_call && num_semis > 0)
	end
	return (needs_parameters=is_paren_call,
	is_paren_call=is_paren_call,
	is_block=!is_paren_call && num_semis > 0)
	end
	opts::NamedTuple{(:needs_parameters, :is_paren_call, :is_block), Tuple{Bool, Bool, Bool}}