Use context.containing_url to track import stack

[ntkme]

This is a major rewrite of importer wrapper to track import stack purely with context.containing_url (a feature added in dart-sass 1.68.0), instead of tracking our own stack with indirections via virtual sass files.

Previously both single ImportResult and multi ImportResult arrays are handled with a virtual scss file for stack tracking. For a single ImportResult, it loads in three files: the first virtual file to "push" into the stack, the second actual file, and the third virtual file to "pop" out of the stack. Multi ImportResult arrays would generate an extra virtual sass file, in which each individual ImportResult loads three files the same way as single ImportResult.

Now single ImportResult is handled directly without having any virtual sass files.

This change has many benefits:

• Robust stack tracking.
  • Fix potentail bugs that native importers like NodePkgImporter breaks stack tracking as they don't go through generated virtual files.
• Better performance.
  • Reduce compiler round trips by removing of majority of indirections from each single ImportResult.
• Better source map.
  • Eliminate two virtual sass files per single ImportResult.
  • Create more readable virtual sass files for multi ImportResult arrays, which now import the resolved paths directly rather than import other virtual files.