fix(sass): reorder sass importers

[Chen-jj]

Description

Currently, the orders of sass's importer options is: [internalImporter, customImporter].

When we import a virtual sass dep like the test case add by #3180, everything is fine.Because internalImporter can't resolved it and then customImporter can handle it.

However, if we import a style file with custom extname like @import './common.wxss';.The internalImporter will resolved it thus there is no chances for customImporter to handle it.

So, we should unshift the customImporters, not push.

Additional context

Test case is added.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[sapphi-red]

Thanks!