breaking: warn on quotes single-expression attributes in runes mode

[dummdidumm]

In a future version, that will mean things are getting stringified, which is a departure from how things work today, therefore a warning first.
Related to #7925 (but not yet closing that issue; we should only do that once we actually have changed the behavior)

Solved by adjusting the AST so that it's now { type: 'Attribute', value: true | ExpressionTag | Array<ExpressionTag | Text>, ... }

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