fix(styling): Add .import sass files

[andrewseguin]

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[crisbeto]

LGTM, but I wonder whether this won't conflict with the changes in #21204. The sass migration script also generates .import files. Will we have to merge the two together or overwrite the existing ones?

[andrewseguin]

I think adding these .import.scss files separately in this PR would be easier since we don't need to worry about fixing google3 issues that arise from it. I had to make a few changes internally since @forward didn't always seem to work transitively. If we can get this in, we won't regress on any future client changes

[jelbourn]

@crisbeto once we can get Google apps switched over to targeting these shim .import.scss files, your PR to adopt @use can basically delete all of these shims and replace them with the real ones. The only catch is that we have to make sure we're forwarding all the same stuff under the same names.

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