Don't add unnecessary placeholder stylesheets

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The placeholder plugin adds a stylesheet with every placeholder widget. See this line. They are injected alongside the placeholder so that the styles will be applied even if the simpla-text instance is being used inside a Shadow Root.

However, this is naïve - instead there need only be 1 stylesheet per Shadow Root that any simpla-text is in. The plugin should check the view.root (which corresponds to the Shadow Root or document that the element lives in) and check if a style has already been applied e.g. via root.querySelector, or by storing it directly on the root e.g. root._placeholderStyles, or by using a WeakMap

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Should be noted that the downside to this is we'd be injecting DOM content into the client's DOM - injecting into the simpla-text light DOM is also running into the client's DOM, but that's less intrusive as we manage that DOM.

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Closing this as it's now a part of simpla-richtext and will be addressed in simplajs/simpla-richtext-behavior#27