message: Refactor changelog for 8.4.2
diff: ## [8.4.2] - 2019-11-27
  +### Added
  +- (#1819) Add an open on the left mode for the viewer tab, controlled by `view.pdf.tab.editorGroup`.
  +- (#1817) Replace placeholders in external build arguments.
  +- (#1777) Declare a new language id for `.rnw` documents.
  +- (#1833) XeLateX live compilation progress.
  +
  ### Fixed
  - Fine tune some log messages.
  -- (#1777) Declare a new language id for `.rnw` documents.
  - (#1793) Broken pdf viewer with inverted color.
  -- (#1817) Replace placeholders in external build arguments.
  -- (#1818) Do not clean `.ist` files.
  -- (#1819) Add an open on the left mode for the viewer tab, controlled by `view.pdf.tab.editorGroup`.
  - (#1822) Output window messes with LATEX Snippet Panel.
  - (#1823) Drop the star when passing `newcommand`s to mathjax.
  - (#1827) `subimport` `path.resolve` issue.
  -- (#1833) XeLateX live compilation progress.
  +
  +### Changed
  +- (#1818) Do not clean `.ist` files.
  ## [8.4.1] - 2019-11-16
  