build: add stylelint rule to validate ampersands in theme selectors

[crisbeto]

Replaces the selector-nested-pattern-scoped Stylelint rule with a new rule called no-ampersand-beyond-selector-start. The new rule will enforce that ampersands can only be used at the start of selectors in theme mixins. This will catch issues like #10889 and #10904 where having an ampersand anywhere else in the selector will work in most cases, but will break down once the mixin is wrapped in a selector.

[jelbourn]

LGTM

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