WIP trying to make sliders active on click

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Current problem:

• Sliders show up for variable fonts, but they can’t be manipulated until the user first selects an instance from the font style menu.
• A user may not know that they first have to select a style, and the may just want to immediately play with the variable sliders.
• If variable sliders show up but seemingly “don’t work,” the extension may seem broken.

Goals:

• Make sliders immediately active on clicking anywhere in the slider area (remove "mute" class), for that given font override section
• Make sliders immediately active on click of any slider (remove "disabled" attributes from each slider), for that given font override section
• Keep the following things working (these currently work well, but only after the menu changes first)
  • Allow variable axes to change immediately, upon slider interaction
  • Update style menu and font toggle title to reflect the change
  • If “Inherit page styles” is selected, re-mute the slider area

This is not done yet, and this PR is an area where I’m tinkering to better understand the system, and hopefully find the solution.

This seems like it should be simple, but it’s a harder puzzle to solve than expected!

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Closed in favor of #144