@import is stripped from the associated .uss file when saving .uxml

How to reproduce:
1. Open the attached "case_1404986" project
2. Open the "Assets -> CardCreator -> cardCreatorV2.5" Visual Tree Asset file in the Project Browser
3. Observe the "cdDefaultTheme" Style Sheet contents under the Style Sheets section

Expected result: "fixUnityStyles" Style Sheet contents are successfully imported to the "cdDefaultTheme" Style Sheet and can be observed under the last one
Actual result: @import is stripped when saving the "cardCreatorV2.5" Visual Tree Asset

Reproducible with: 2020.3.38f1, 2021.3.9f1, 2022.1.16f1

Couldn't test with: 2019.4.38f1 (the project isn't compatible)

Already fixed in 2022.2.0a15 but needs a fix in 2021.3 and 2022.1.