Scope variables in Custom PostCSS plugins to each file processed

[MadLittleMods]

As discovered in #722 and suggested we should that we should still put some effort into fixing:

[...] could you still have a look to move the scope of those variables down? I think it's a good thing to do regardless of it not being an immediate problem anymore now.

-- @bwindels, #722 (comment)

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Scope variables in Custom PostCSS plugins to each file processed instead of in the global plugin instance.

We should move the map variables inside the file processing (Once) scope so they aren't shared across plugin runs. [...] We should probably also pass them around directly to avoid concurrent parallel processing problems.

Example: https://github.com/vector-im/hydrogen-web/blob/480c5c1584c43c0f2d1ad82054e33b62a442a730/scripts/postcss/css-compile-variables.js#L136-L139

Currently, this doesn't cause any issues in the codebase. But this is a bit of technical debt that we should get to at some point.