spec: revisit syntax for index expressions

[griesemer]

The section on Instantiations doesn't have any syntax associated with it, it relies on operands with type arguments.

The section on Index expressions relies on the Index production but that production allows for a comma-separated list (which can't be used for indices). The reason is that the parser uses index expression to also parse type arguments.

This needs a cleanup; we should be able to present this more clearly in the spec.

[gabyhelp]

Related Issues

• spec: document that `x[i,]` (trailing comma) is permitted #55007 (closed)

Related Code Changes

• spec: factor out notion of index values into separate section
• [dev.typeparams] Handling multiple type arguments for call via new node OLIST

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