Vim script syntax highlighting

Added the "vscode-viml-syntax" (1) extension for syntax highlighting in
all Vim script files.

References:
  (1) https://marketplace.visualstudio.com/items?itemName=dunstontc.viml

Resolves GH-184

snowflakes/visual-studio-code/extensions.json

@@ -4,6 +4,7 @@
   "christian-kohler.path-intellisense",
   "dbaeumer.vscode-eslint",
   "drewbourne.vscode-remark-lint",
+  "dunstontc.viml",
   "EditorConfig.EditorConfig",
   "Equinusocio.vsc-material-theme-icons",
   "esbenp.prettier-vscode",