Reduced memory retention causing high CPU and memory usage

[Harkarman]

Summary

Reduce long-lived memory retention in the Import Cost extension and calculation pipeline.

• Dispose VS Code document/editor event subscriptions through the extension context.
• Debounce document edits to avoid starting a full calculation on every keystroke.
• Clear pending document and decoration timers during deactivation.
• Release per-file emitters and cached decoration data after calculations complete or the extension deactivates.
• Remove settled debounce promises from the in-memory key map.
• Delete temporary Webpack entry files and close Webpack compilers after builds finish.

Close #342
I expect it will also close #341 , #340, #338, #331. Didn't check beyond that.