feat(ungrammar, codegen, css_parser): support "dynamic" unordered node fields

[faultyserver]

Summary

This PR implements the idea behind #1407, which is the ability to define, parse, represent, and interact with unordered fields on a node.

Without repeating too much of that discussion, the idea is that the grammar can define a set of fields that are unordered: each field can appear at most once in the node, but they can appear in any order in the set. In an AST, this is simple, but in a CST we need to be able to map the access for a specific field into the ordered, linear stream of tokens and nodes of the untyped syntax tree.

To do this, AstNodes with one or more unordered fields also include a slot_map member that maps a defined slot index (the index of the field in the grammar) to a concrete index (the index of the field in the source code token stream). When converting from an untyped node to a typed node, the constructor builds the slot map by comparing the children against the specified grammar and assigning the slots. This also happens when up-casting the raw token stream to the SyntaxNode, but only for validation and filling empty slots to ensure missing and required empties are filled.

Test Plan

There's nothing that uses this yet, but I made a sample grammar with unordered nodes, ran codegen, and tested a parse result that you can see in this commit.

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[github-actions]

Parser conformance results on

js/262

Test result	main count	This PR count	Difference
Total	49701	49701	0
Passed	48721	48721	0
Failed	980	980	0
Panics	0	0	0
Coverage	98.03%	98.03%	0.00%

jsx/babel

Test result	main count	This PR count	Difference
Total	40	40	0
Passed	37	37	0
Failed	3	3	0
Panics	0	0	0
Coverage	92.50%	92.50%	0.00%

symbols/microsoft

Test result	main count	This PR count	Difference
Total	6322	6322	0
Passed	2036	2036	0
Failed	4286	4286	0
Panics	0	0	0
Coverage	32.20%	32.20%	0.00%

ts/babel

Test result	main count	This PR count	Difference
Total	662	662	0
Passed	592	592	0
Failed	70	70	0
Panics	0	0	0
Coverage	89.43%	89.43%	0.00%

ts/microsoft

Test result	main count	This PR count	Difference
Total	17646	17646	0
Passed	13452	13452	0
Failed	4192	4192	0
Panics	2	2	0
Coverage	76.23%	76.23%	0.00%

[ematipico]

What should we do in case we mistakenly reach this code path?

[faultyserver]

This should be genuinely unreachable, since we break out of the loop before parsing a second rule if we encounter a non-combinator, and we also bail the parse entirely if we encounter mixed combinators, so I think this is safe as-is. Reaching this state would mean there's both a bug in the parsing logic above it and an unexpected input.