nes: refactor prompt construction for extensibility (#1241)

* nes: refactor: get config from within computeEditWindowLinesRange

* nes: refactor: tagging

* nes: refactor: extract tagging current file into its own method

* nes: refactor: keep active document props in a single object

* nes: refactor: extract cursorLineOffset

* nes: refactor: more refactoring

* nes: extract language context computation

side effect that we now always await not when language context is enabled

* nes: refactor: move CurrentDocument to common/

* nes: refactor: abstract constructing messages

* nes: fix off by one