fix: disallow sequence expressions in @const tags

[dummdidumm]

closes #11349

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[jrmajor]

This will throw an error for {@const foo = ('bar', 'baz')}, which I don't think should be forbidden. I've also started working on this issue and created a test for that, but don't really have an idea how to get it to pass, because parentheses are not present in AST. Feel free to cherry pick it from 495c1a5 :)

[fry69]

It is a bit weird to see the comma operator getting used outside a for loop, but it is valid JavaScript.