prepend popover stylesheet

[keithamus]

Description

This may go some of the way to fixing #147 which is still an issue for users, despite #153 which dropped selector specificity down to 0,0,0. Right now if a selector is used that has a specificity of 0,0,0, the polyfill will still potentially override it as the popover styles get appended to the head/adoptedStyleSheets, and as such will be lower in the cascade.

This change ensures that the popover stylesheet is prepended rather than appended, which means it will appear as high up in the cascade as possible, rather than potentially at the bottom.

Steps to test/reproduce

N/A

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