[dart-misc] Dart Code v1.4.0 - improved code completion and code assists (lightbulbs)

[Danny Tuppeny]

View on the VS Code marketplace here
<https://marketplace.visualstudio.com/items?itemName=DanTup.dart-code>.

I'm specifically interested in feedback around the code completion changes;
I don't know if the increased frequency of updates may introduce new issues!

- Code assists are now available and will show up as lightbulbs similar
to existing code fixes (for example, `Wrap in new widget` in Flutter
projects)
- Code completion will now refresh on certain key presses (`space`, `(`,
`=`) which should result in more accurate results more often
- Code completion results are now sorted by relevance rather than
alphabetically
- Cursor position should now be better maintained when executing code
fixes
- `pubspec.yaml` and `analysisoptions.yaml` will once again use the
built-in YAML language/highlighting
- When opening a flutter project, the `.packages` file will now also be
used when trying to locate the Flutter SDK
- The error message shown when unable to locate the Flutter SDK has been
simplified and a link to the Flutter download page added
- The environment variable `FLUTTER_HOME` is not recommended for use and
will no longer be used to locate a Flutter SDK (`FLUTTER_ROOT` is still
checked)